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FAQs about Specific Gravity, Salinity Measure

Related Articles: Specific Gravity, Salinity, Product Review Marineland Labs/Aquarium Systems Hydrometer, Part 1 By Steven Pro, Choosing Synthetic/Natural Seawater, Major/Minor Seawater Constituents, Frequent Partial Water Changes

Related FAQs:  Spg 1, Spg 2, , & FAQs on Spg, Salinity: Importance, Science, Maintenance, Anomalies, & Treating Tapwater For Marine Aquarium Use, Seawater, Seawater 2, Seawater 3, Seawater 4, Seawater 5, Seawater 6, Reverse Osmosis Filtration, Test GearUsing Hyposalinity to Treat Parasitic Disease,

Salt concentrations in ppt (parts per thousand) and ppm (part per million) to percentage readings http://edis.ifas.ufl.edu/VM007 http://www.koiclubsandiego.org/SUBcategory.php?categoryKey=3&subCategoryKey=19&subCategoryName= Salinity&PHPSESSID=f7a64e77d6b7b88b6b21f06b6b31ffb3 Hydrometers... floating (glass, generally is least accurate), box-type... Conductivity (meters, pens),  Refractometers, more accurate and precise...

Accuracy... a measure of what actually is Precision... the capacity to replicate a measure

Re: Sailfin Tang Heavy Breathing... Now spg vs. salinity, conductivity...     2/18/19
Hi Bob,
<Hey Charles>
Regarding your point about the difference in salinity and specific gravity - in your opinion, is it worthwhile purchasing a conductivity meter to have a second measurement? Would this be useful in identifying potential issues when the readings don’t match?
<Mmm; possibly... This general statement re: GIVEN folks doing frequent partial water changes (10-20% a week let's say), NOT having too much "other" sources of dissolved solids (e.g. foods/wastes, dissolving substrates, supplements...) utilizing a simple hydrometer or (better) refractometer and relying on it solely is likely fine>
<And you, Slippery Slope BobF>

Refugium setup- sg variations  10/28/13
Hello Mr. F,
I am glad that you think that my refugium is all right. I can`t wait to see it working. By the way, how long do you think it will be until the DSB starts functioning ?
<As soon as it's up and running>
I have another question: I usually try to keep my salinity at 35. I have an AquaMedic auto top off installation, installed in the refugium but I get during 24 hours swings of about 0.8 units. So it goes down to 34.2 and up to 35. I don`t really understand why this is happening,
<How are you measuring salinity here? It may be that the difference is simply due to the measuring device, temperature>
maybe it has something to do with some minor fluctuations in the water level because of the filter socks that I have installed on my DT drains. I change them every 2 days, but as they get clogged they retain more water that is subtracted from the general level. What do you think about these variations? Are they dangerous?
<Not likely, no. Probably don't exist>
Please keep in mind that my tank is SPS dominated and my plans are in that direction, so I`m talking Acropora sensitivity.
<Check the density of the water via a decent refractometer... Turn of the ATO if concerned>
I also have a fairly large amount of water in the system, probably 300 g so maybe the top off has an inertia?
<Interesting question... shouldn't>
Thank you,
<And you, Bob> 
Re: Refugium setup- sg variations      10/28/13

Hello and thank you for your answer.
I am measuring the salinity on my apex Neptune equipped with the salinity module.
<Good units; and a very nice, competent fellow owning, running the company... But I would still check, calibrate this against another device... the aforementioned refractometer would be my simplest choice. B>

Seawater Density      6/7/13
I recently picked up Chris Brightwell's book on Marine Chemistry and have found it quite useful. However, after reading chapter 3 I became slightly confused. Chapter 3 deals with Salinity, and in that chapter, Mr. Brightwell states that the average salinity of seawater is 35 and that it corresponds to a density of 1.024. However, when I look at my refractometer, and the way the 2 scales are lined up, a salinity of 35 lines up with a density of 1.027. Could you shed some light on this confusion for me?
<Mmm, likely the difference here is accountable as a thermal, calibration issue. Density of seawater varies per temperature and measuring devices (refractometers, hydrometers...) are standardized per a stated temp. Use your search tool/s with the string: "seawater density salinity and temperature"
and read a bit>
<Welcome. Bob Fenner>
Re: Seawater Density     6/7/13

Thanks. Mr. Brightwell brought up that connection in his book as well. Is it safe to say that the average salinity of seawater is 35, but the average density of 1.024 is actually brought down
<Mmm... no... cooler water is more dense; and this statement is too teleological to suit me>
 by the cold waters of the northern and southern most regions of the ocean? If that is the case, is the average density of the reefs around 1.026,
<Is about this at tropical temperatures>
 and where would you recommend maintaining the density of an aquarium at ~77 degrees?
<1.025-1.026. This is gone over on WWM. BobF>

finding and selecting low range hydrometers, BR and SW f'     2/5/12
Hi WWM Crew,
Your site is pretty much my aquarium bible. Its great! And i have recommended it to a number of workers in my LFS as well as friends. Especially after I overheard some really awful advice being given to a customer at which point I kind of inserted myself into the conversation and offered some advice. Probably should have minded my own business but the clerk obviously had never heard of the nitrogen cycle or prime (which was right there on the shelf) and was selling a bunch of fish to a guy to put in a tank far too small for the number and size of fish he was purchasing. I had to speak up, as politely as I could, but i did manage to educate both of them a bit. I'm no expert, and a year and a half ago i was clueless, but I have studied and read and read and read and now i feel like I'm probably approaching the intermediate level of freshwater fish keeping. I now also have a low to medium salinity brackish tank with only Black Mollies (2 females and 1 baby which has turned out to be a Dalmatian and is now big enough, about 1/4 to a 1/2 inch long, so it wont (shouldn't) be eaten. All 3 are doing very well in this 10 gallon tank. The tank was bought to be a quarantine tank, but I couldn't leave it empty, so after a month of fishless cycling and finally getting the Am and nitrites down to 0 and also, just to be sure, i added a little pure ammonia and watched to see how it was handled, , it disappeared pretty quickly, nitrites briefly bumped up a little and in a day or 2 i was back to Am=0, Nitrites=0 and some nitrates. I then moved 2 mollies from my freshwater tank to this 10 gallon and over time have worked up the salt . I currently use 3 level tablespoons (the measuring spoons, not the food spoons) of Instant Ocean per 10 liters of water with a tiny bit of prime (1/8 cap or so) per 10liters of water change. The tank came with a whisper 10 HOB filter which in my opinion is not adequate for a 10 gallon tank, but rather than replace it with a Whisper 20 I bought a 2nd Whisper 10 to run in parallel, now i can alternate filter swaps and not upset the nitrogen cycle too much. Also, and this is cool, i found 2 cheap sponge filters that fit nicely on the whisper intakes and provide a little more area for nitrifying bacteria to grow in along with keeping any big particles out of the filter. They work great. I just buy empty filter cartridges, I don't use carbon unless I see a particular reason to use it for a week or so. The tank has plenty of plastic plants and a cave, so its not too plain. I also have a large round (4" diameter) air stone which the mollies love to play in, its funny, they actually will swim over the stone and allow it to float them up, then swim out and do it again, after awhile they get bored and go off to do something else. Temperature is 80F, Am is 0, Nitrites are 0 and well, you know how nitrates are, they always climb, but i try to keep them below 20mg/l. I want to measure the SG using a glass hydrometer , the trouble is that the scale i am interested in is 1.000 through say 1.010 or possibly 1.015and that tends to be very low on the hydrometer scale. I just don��t trust any instrument that��s reading close to one end or the other of its full scale. With that in mind I went looking for a suitable hydrometer. They are quite hard to find but I did find these three: http://morebeer.com/view_product/18650/102224/Hydrometer_-_Final_Gravity  http://www.austinhomebrew.com/product_info.php?products_id=2089
They are used primarily in beer and wine brewing . I was leaning towards the first link, the 3rd link seems to have a lot of choices but its nearly impossible to understand what the ranges are, at least for me its extremely confusing. All of those in the 3 links above would need to have their readings temperature corrected. Can you comment on what would be a good choice for my application? Are they even appropriate if i adjust the reading for temperature? If not, what do I do, i suppose i could mix, measure and call it "close enough" but I'd rather know what I really have in there. One more thought, I really noticed a striking change in the mollies when they got into the salted water, they seem much healthier. When they were in the freshwater tank, I had one that had some white stuff on her belly and an especially large glob near the anal fin, I couldn't diagnose it, I watched it for over 2 months, it wasn't Ich, the other molly is coal black and if it was Ich this would have spread. My guess was Columnaris, so i treated the tank for it. It didn't help a bit. But after I moved her to the new tank and brought up the salt (marine salt) level I noticed a distinct change and a definite improvement, so whatever it is, its getting better around the anal fin and as to her belly? Well, maybe she's half Dalmatian. I really don��t know but her baby is definitely a Dalmatian. I'd send a picture but there must be some magic to getting a good shot, I just can't get a decent photo. I need to say one more thing for those reading your site, use a maintenance log program, and log every single thing that you do, measurements, fish deaths, stuff added from chemicals, to salt to plants, lights changed, water changes and how much (I use a bucket with 5 and 10 liter marks on the outside as it seems like almost everything in the fish industry that matters is metric), log it all, it's worth its weight in gold when you can look back and see what is really going on i your tank, not just what you think you remember. I have a log for my 10 gallon and a separate log for my 44 gallon tank. I use Aqua Log, its free and if you set up the columns before adding any data it will be nice and orderly and its simple to use.
<Hello Eric. The short answer is that any hydrometer that runs from 1.000 to 1.030 is going to be useful for fishkeeping. However, a couple of issues. First, it needs to be calibrated (i.e., accurate) at a useful temperature, e.g., 20 or 25 degrees C. If the calibrated temperature is far above or below these, it's less useful. Second, any hydrometer (or refractometer) is only as accurate as its design allows, and consumer grade ones costing a few pounds or dollars will be less reliably accurate that scientific ones costing tens if not hundreds of times more. The best way to be sure is to make up a known concentration (e.g., 35.5 grammes marine aquarium salt in 1 litre of pure/RO water) at the calibrated temperature (e.g., 25 C) and then test the hydrometer. If thoroughly dissolved, which may take 20 minutes or longer, 35.5 g/l should be 1.025 at 25 C. If your hydrometer is far above or below that, then make a note of how far, and adjust accordingly when using the hydrometer. To be honest, brackish water fish largely don't care, so if you're dosing at 5 grammes/litre, an ideal amount for Mollies, you should register SG 1.002-1.003 on your hydrometer, but if its a bit above or below, no worries! Go with salinity (grams/litre) over hydrometer reading any day of the week. Fish experience the salinity; specific gravity is immaterial. Cheers, Neale.>

Refractometers/Powerheads/Selection 12/21/11
<Hi Joe>
With so many on the market which one is most reliable and easy to use and calibrate??
<Many of these Refractometers are made by the same manufacturer for several companies.  I personally like and use this model. 

Re Refractometers Selection 12/28/11
<Hi Joe>
I'm trying to order refractometer from jlaquatics. They carry 2 one is Vertex the other is called salinity refractometer. Which is best? Or there's sybon from bigals. Very limited review info. Very limited on selection in Canada. Pls advise.
<I don't believe there is a great deal of difference between refractometers other than the quality of the prism.  If the Vertex isn't considerably more than the others, I'd go with the Vertex.
A refractometer works on the principle of refraction, that is the bending of light waves as they pass from one substance into another....nothing electronic about it.  The saltwater sample is sandwiched between the illuminating prism and the refracting prism.  When held up to a light source the refractometer measures the degree of light bending and assigns a number to it. The number is known as the refractive index and will be displayed as specific gravity.
<You're welcome.  James (Salty Dog)>

Fluctuating salinity! 2/13/11
Hi guys
<And gals Gemma>
Bit of a strange one that I hope you can shed some light on...
<Will try>
I have a 94l reef tank which is currently 4 weeks old. It is one of those all-in-one tanks with compartments on the back and they are filled thus:
1. Boyu WG310 protein skimmer
2. Eggcrate rack currently with live rock rubble on one shelf and Rowaphos
<Why this last?>
on another (in a filter sock with filter wool around it)
3. Heater
4. 2x 1200l return pumps
My param.s all seem to be stable except for the salinity which is all over the place and I've no idea why! It increases by about 0.5
on a day-to-day basis which I understand is due to evaporation and this is fine, I top off with RO to bring it back down. The last two weekends when I have been water changing I have been getting some very strange readings. I use a brine refractometer
so aim to keep my salinity at 37 as it's not a seawater refractometer. Last Saturday I tested the water in the morning and it was at about 37. As I was going between testing my tank and the bucket of water change water readings my tank AND bucket salinity started falling, resulting in my tank apparently ultimately hitting 31 salinity according to my refractometer. The salinity in the tank then seemed to rise to about 34 and I started adding salt to the bucket to get it up to 35 in order to drip into the tank to bring up the salinity slowly. By the end of the night the tank was apparently up to 36 again which by rights should have caused the inverts in my tank a lot of stress - but everything seemed fine. Following this palaver the tank stabilised back at 37 and I put the readings down to user error.
Sunday-Friday this week I was back to plain old salinity testing once a day and every time the salinity was in the expected range of 37.
Today came to water change and this morning my tank was on 37.5 so about right. I tested the bucket of water change water (which has been mixing for about 24 hours, same as last week) and found it to be 36 so I added a small amount of salt to the mix and went back half an hour later to find the mix had dropped to 34!
In another panic I tested my tank again and it had dropped to 35, then later on 34! I left it for a while and now it is back up to 36.
<... gear>
It HAS to be user error, I'm sure - nothing I know of makes salinity drop aside from salt creep and possibly wet skimming but as far as I know they can't explain why it seemed to drop by 6 points last week within a couple of hours. It only happens on water change days when I am repeatedly testing to get the match on salinity between the old and new water, on normal days when I just test once the reading is always within the expected range. I am wondering if it is something to do with testing then testing again and
there being some residue on the lens or something?
But in all honesty I don't see how... I put a few drops on the lens, put the cover down, read it under light, wipe it with a clean, damp bit of kitchen roll then dry it off with a dry bit. The refractometer automatically temperature compensates but either way my house is pretty much consistently 18 degrees so doesn't fluctuate enough to cause this...
Please help, I am desperately worried that I am stressing out my new inverts and have no idea what to do!
Thanks in advance
<I'd get a seawater refractometer. Bob Fenner>

Re: yo-yo nitrates, fish death. Hydrometer sel.  2/11/11
Hello Bob,
I just wanted to thank you for the all of the advice, and to update you on things. Things have been going well in the tank. The new rock looks great and has added a lot of life to the tank. All levels have been good and my nitrates have been hanging in the 10 ppm area with 10% weekly changes. I have two little neon gobies and as they seem to be happy I am going to start QT on my next fish this weekend. I do have a cautionary tale for people.
I discovered before I got my new fish that my salinity was off. I took a water sample to the LFS and they told me it needed to come down. My hydrometer had been telling me it was at 1.023-1.024. It was at 1.028. I bought a refractometer that weekend. I don't know if that killed my first two fish, but I imagine it contributed.
I am sure you already caution people about the reliability of hydrometers,
<Indeed I have. Most "cheapy" glass ones are neither accurate nor precise>
I just thought I would add my story for
others to learn from. Thanks again for all of your help.
<Thank you for sharing. You have and will help many others. Bob Fenner>

Pinpoint Salinity Meter   1/16/11
Hi guys,
I really appreciate all of the responses that I have received so far.
Great info and really helped me allot.
<More than this allotment I hope/trust>
I purchased an American Marine Pinpoint Salinity meter about two weeks ago.
After calibration I began testing the water. I found that the water in my tank had a significantly lower specific gravity than what my hydrometer was reading.
<Does happen>
Just to be sure that the device was accurate, I decided to test it on RO/DI water. The reading was only 0.1m/s. So, I believed that the salinity meter was obviously correct. I increased the salinity (by 0.3 specific gravity) slowly over a week.
<Mmm, a good idea to check your meter against/with another accurate device... perhaps a decent refractometer... And/or a known sample (for SpG) of seawater>
However, I noticed some mushroom acting odd about 3-5 days after the salinity change. This made me worry that my salinity meter was in fact "off." So, I dug up a different old hydrometer and tested. This hydrometer also showed a much
higher reading. Although, not as big of a difference as the previous one.
So, I then proceeded to check the salinity of the NSW in my area (San Diego, CA).
<Oh! Water from SIO? I too generally live in this part of S. Cal. (out in Cozumel currently)>
My salinity meter read 53.0m/s (specific gravity of 1.026) which sounds perfect!
<Oh yes>
Here is the interesting little glitch. So did my hydrometer! (it was slightly higher, but only a bit) Now, I was thoroughly confused. I thought it could be from electrical interference.
<Mmm, not likely... but you could (I would) test the SpG with a sample outside the tank (like in a glass)>
So, I took out a cup of water from the tank and tested it outside of the tank. The readings were the exact same as within the tank.
<Ah, good>
Although this is more of an "experience" than a question, I thought you guys might also be able to provide some insight into this. I will start you off with a possibility that I thought. Are the calcium, magnesium and other ions messing
with the hydrometer/ salinity meter?
<Mmm, shouldn't, not appreciably>
How? & what instrument should I trust?
<More the meter... IF it has been established to be calibrated>
Tank info:
Salt: Tropic Marin Bio Actiff
Ammonia, Nitrite, Nitrate =0
PH= 8.2
Temp: 78'F
Specific Gravity = Who Knows! (salinity meter has it at 1.0256 specific gravity)
Calcium: 450ppm
Magnesium: 1350ppm
<Mmm, these are a bit high, but s/b okay>
<Thank you for sharing John. Bob Fenner>
Re: Pinpoint Salinity Meter   1/19/11

Hi Bob,
<Hello John>
Thanks for the response. Yes, it was in fact the Scripps water.
<Ahh! It and I share quite a bit of co-history>
This whole thing seemed rather perplexing. However, after more testing and watching, I have decided to stick with the meters results and let it be.
<This/that is what I would do as well. Cheers, BobF>

Specific gravity and altitude.    07/20/09
Hi team,
<Hello Keith,>
whenever I cannot find the answers to something I revert to you guys. Hope you can help.
I am curious to find out what effect altitude will have on the specific gravity of a salt water solution.
<There is a very, very slight effect within the range of atmospheric pressures; reducing air pressure reduces specific gravity if temperature and salinity stay the same.>
Now I know that pressure and temperature affect the SG so here is the scenario.
A salt water sample is measured at 1,026 at sea level where the atmospheric pressure is approximately 14.7psi. The same sample is then measured at an inland town at an altitude of 1800m and an atmospheric pressure of 12.27psi. All measurements at a temperature of 20C.
1) What is the difference in SG at the 1800m altitude?
2) How is the difference calculated and what formula is used?
3) Please do an example using the above information.
<It's actually a hugely complex calculation, and not at all easy to do.
When creating my program "Brack Calc" that converts between salinity, specific gravity, and temperature, I found the mathematics complicated enough just with those three variables.
Unless Bob or someone else happens to have a rule of thumb for this, I'd say either ignore it, or else go find a physical oceanographer who'd actually be able to work through the mathematics with you. I will make a comment with regard to the latter, if you do approach a scientist, remember not to ask them to do your work for you, but to help you do the work yourself. As a scientist myself, I will tell you this tack always works better!>
The second part to this subject is what effect does the altitude have on the accuracy of the measuring equipment. I have done the experiment of placing a hydrometer in a plastic cool drink bottle partially filled with sea water and sealed the top. When you squeeze the container the SG increases and if you apply a vacuum it decreases. So what are the effects of the following equipment. And how is it calculated?
<Damned if I know. I do marine biology and oceanography at university, but the air pressure factor is so trivial compared with water depth and temperature that it is not really of importance, so we didn't learn about it. There's obviously no sea at the top of Mount Everest, so being able to figure out the density of seawater at that altitude is not of any real value.>
1) A refractometer with ATC
2) A professional glass tube hydrometer similar to the tropic marine one with an accuracy of .0001%
<Normal seawater is 35 grammes per litre; you could start by making a ten one-litre batches of seawater using a freshly opened box of salt mix.
Record the density of each at precisely 25 degrees C, which is where specific gravity is calibrated from. Take the average, and off you go! Note the importance of using anhydrous sea salt mix to avoid moisture from the air reducing the amount of actual salt per 35 gramme portion. Temperature us also critical, and has a far bigger impact on density than air
3) A conductivity meter.
<Conductivity is how marine biologists and oceanographers measure salinity, so if you have such a tool, may as well use that. Conductivity isn't affected by air pressure, so far as I know.>
The third part is what effect does the altitude have on the fish and corals at this altitude?
<Minimal, given water is largely non-compressible, so the affect of air pressure on density/specific gravity will be extremely slight.>
Do you have any other links/references on the subject.
Many thanks
Nemo's Janitor.
<Cheers, Neale.>

Broken Glass Hydrometer 6/9/09
Sorry to bother you but my son was helping set up my 72 gallon sw tank with live rock and broke the hydrometer. I recovered the glass but the black particles from the bottom of the hydrometer/thermometer is hard to get out.
Like I said there is nothing but live rock in the tank. What will this do and do I have to start over?
<Nothing to worry about, no harm done.>
Valerie Jones
<This happens quite often, nothing to worry about. Do consider an investment in a refractometer for future tests. They are very accurate and much easier to use. Scott V.>

Refractometer question 04/25/09
Hey there "Crew", a quick and easy question for you.
I don't use RO/DI water for my tank, I have no easy access to "pure" water except what comes out of the tap.
<Where do you live? Most drug and grocery stores in the US sell distilled water.>
What I did to calibrate my refractometer is boil water. The steam collected on the lid of the pot, then I quickly picked up the lid and turned it sideways, which caused the steam droplets to intersect, form a large water drop and drip off onto my refractometer.
<Clever... and this might be good enough. The droplet you got from the boiled water is not "pure" (b/c of contaminants on the lid itself for one thing)... but it might likely have a salinity quite close enough to 0.>
I then adjusted my screw so that it read as 1.000 salinity.
<I think/hope you meant "0.000." Distilled/pure water has 0 salinity.>
Does that sound like an accurate way to gauge or do I really need to use RO/DI water to be accurate?
<Seems good enough, if you set the home "distilled" water to 0.>
Sara M.>
<I'm sorry... I goofed. If you are calibrating a refractometer that measures in SG, then yes, you were right, you set the baseline to 1.000...
if in PPM, then it is set to 0.0. Sorry about that...
Sara M>

Refractometer cover plates 11/13/08 Hi Bob, I met you at the House of Fins Anniversary Splash Event! Sat. Oct. 18th and have a mutual friend at HVRK...Linda C. <Ah, yes> I've searched just about every where on the web before Id bother you but your my last resort. I'm looking for a replacement cover for my RHSN-10ATC refractometer. Can ya help me out there bud?? Sincerely, Rick Sharp...The carpenter...remember?? hee, hee , hee <Sorry for the tardy reply Rick... have been OUT! I would try the folks at Reef Solution: http://www.reefsolution.com/catalog/product_info.php?products_id=1281&language=en Bob Fenner>
Re: Refractometer cover plates  11/14/08
Thanks Bob but it was a no go. Seems they just don't sell the cover plates separately. <Mmm... how about writing the company directly then? Http://www.sinoptics.com It may take some doing... but can be sent from China... or they may know of a distributor that is more accommodating> How are thing with you?? <Fine... just a bit jet-lagged> I just bought your book, The conscientious Marine Aquarist...Whew, that second words a doozy...lol <Means "with knowledge" basically... I did "make up" the title... in part in reaction to the "Dummies", "Idiot" words used in some other series...> Next time your out this way I'll have to trade you autographs. Sincerely...A fan Rick <Do make it known what you can find out re the Refractometer repair Rick. Cheers, BobF>

Plastic type hydrometers  9/5/08 Good morning Bob, <James! Howsit?> Just a note to pass on to marine aquarists who are using the . I purchased a refractometer with automatic temperature compensation, calibrated it to 35ppt/1.0259 with American Marine calibration solution. I then checked the SG of my reef tank and was amazed I was only reading 1.019 versus the 1.024 of the Instant Ocean hydrometer. <Mmmm> I double checked the calibration of the refractometer and checked the water again getting the same reading. I would like to suggest to people using plastic type hydrometers to find a dealer or friend that has a refractometer and compare readings and note for future reference. Although 1.019 isn't a dangerous level, it certainly isn't a good level for many corals. Have a nice day. James (Salty Dog) <Do agree that "box" types hydrometers have had troubles with accuracy AND precision... especially when they are "new"... it pays to "tap" the box, with water/sample in it... to dislodge bubbles from the "arm". Cheers, Bob Fenner>

Newbie question...please help!! Hydrometer reading/calibration for temp.    8/11/08Hello <Philippe> I hope all is well with you. <Yes. Thank you> Newbie question...because I am soooo confused. I am setting up a 75 gal reef tank. I want to get off to a good start...and I find myself confused. This morning, using an Instant Ocean hydrometer, I had specific gravity readings of 1.0245 at 75.8 F. How does that sound? <Okay....> Is the Instant Ocean hydrometer giving a temperature corrected reading? <Mmm, no... is given a temp. calibrated reading...> Or, is it necessary to use a conversion of .0021 to have a temperature corrected reading of 1.0266 (based on John Tullock's book _Natural Reef Aquariums_ pg. 121) thus a salinity reading of 36? <Yes> Wouldn't that be a bit high? <Not too high... is "right about right"> Anyway, lights will be added tomorrow. I am taking each step slowly and want to get the salinity correct. Thanks for your time. _The Conscientious Marine Aquarist_ has been an excellent source...great book!! -Phil in Mississippi <Ahh, thank you for your kind words. Bob Fenner, presently out in Kona, HI>

Kalkwasser and Specific Gravity -- 02/04/08 Hi, WWM Crew. <<Hiya Al>> Great site and thanks for all the past help. <<Glad we could be of assistance>> I have a question regarding salinity, specific gravity and Kalkwasser. <<Okay>> I dose Kalkwasser through my auto top-off in to a 180g reef tank with a 50g sump. My evaporation rates are fairly large and I was wondering what effect Kalkwasser had on my specific gravity reading. <<Nothing of significance>> Specifically (no pun intended), could an abundance of added Kalkwasser contribute to an increased level of specific gravity? <<Not as I am aware. I have heard before some hobbyists reported 'small' increases in Salinity with the use of Kalkwasser'¦but I haven't seen any scientific claims re, nor have I experienced this with my own tank (that I could tell, anyway). I think some of these 'reports' may well stem from the use of commercial 'two-part' Calcium and Alkalinity supplements as these do impart both Sodium and Chloride ions to the system which would contribute to an increase in Salinity over time. But all in all, even with such additions, the increase in Salinity should be very gradual and easily managed through normal good maintenance/husbandry practices>> Thanks again! Al Lake Tahoe, NV <<A pleasure to share. EricR'¦Columbia, SC>><Specific gravity increases with any solubilized solid material... RMF>

Re: Unintentionally Kill New Finger Leather?, & Spg measure...  12/25/07 Thanks a bunch Bob! <Daryl> I thought you would like to hear some good news... <Always> The "finger leather" is still alive... and is beginning to extend its polyps! (see Finger Leather.JPG). Also, the "kenya tree coral" (?) has shed all its mucous and looks just great! (see Kenya Tree Coral.JPG). I've already noticed a reduction in algae after switching to RO/DI, adding a Turboflotor skimmer, and adding a Yellow Tang! Everyone seems to be doing great! My coralline algae is starting to peal and turn pale in spots... I suspect replacing the light bulbs and the glass covering over the tank has something to do with it. <Likely so> Also, just a quick question regarding specific gravity/salinity. I have a plastic, swing-arm type hydrometer (Deep Six Hydrometer). It is stated that it gives temperature-corrected readings in warm water aquariums. The hydrometer shows a bracket (which I assume they are marking 'normal readings') between 1.020 (~27 ppt) and 1.023 (~31 ppt). I am under the assumption that I should have a specific gravity of 1.024 (~33 ppt)??? I've also read that natural seawater is around 35 ppt (~1.026). I think I'm making this a bit too confusing... basically, what should I be reading on my Deep Six Hydrometer? <About 1.026> Also, my pH is around 8.2 (two hours before lights out). I would like Is there a way to increase the pH without affecting the alkalinity? <Posted... Read here: http://wetwebmedia.com/marine/maintenance/index.htm scroll down...> Water Specs: Salinity 33 ppt Temp 78 pH 8.2 Nitrate 0 Ca 430 Alkalinity 3.5 mEq/L Mg 1500 Thanks for all that you have done. I hope you had a very Merry Christmas! Also, when is your 'reef book' coming out? Daryl <I wish someday soon... no scheduled production time... but I do keep bugging JamesL, my US publisher re... Cheers, Bob Fenner>

Refractometer 07/27/07 Hi Crew I have a quick question if I could bug you guys again. I bought a refractometer after reading some of your posts about hyposalinity. Just in case I need to use it for ich in the future. My refractometer is off quite a bit compared to the swing arm hydrometers I have. I have 2 deep six by CoraLife hydrometers and a SeaTest hydrometer. All 3 tell me my SG is 1.020 and my new fancy dancey refractometer is telling me my SG is 1.024. Big difference. I calibrated it as per directed. I checked seawater from the Gulf Of Mexico off the west side of Florida and the refractometer tells me the gulf is 1.030 SG. Is that normal SG for the ocean or would you say my refractometer is off? I checked the gulf water with my 3 swing arm hydrometers and they are almost at 1.024 just a tad under. The funny thing is I bought this much more expensive refractometer to be more precise and I think my swing arm are more accurate. Anyway to calibrate it I was wondering if you knew what the Gulf Of Mexico SG is? <Probably somewhere around 1.027. However, you should probably account for a possible/likely rise in salinity due to evaporation during the trip from the gulf to wherever you are.> I did a google search and couldn't find anything? Also for what its worth the refractometer I bought is temperature self adjusting. Thanks In Advance. <I would trust your refractometer over your hydrometers (especially if you recently calibrated your refractometer. In fact, hydrometers are usually calibrated against refractometers. Best, Sara M.>
Re: Refractometer 10/28/07
Thanks so much for your fast response! There was no evaporation because I checked it right at the gulf shore. 1.030 every time. <Oh, huh...> What I was going to do is use gulf water and then calibrate at 1.027. I thought 1.030 was way to high. Maybe its a cheap one I bought? <Well, how cheap was it? Did you pay in quarters and have to turn a knob until it came out the bottom of a machine? :-) j/k The "cheap" ones usually work fine. I think it just needs to be calibrated. But I wouldn't use water from the gulf since we don't know exactly what that salinity is (my guess is it could be anywhere from 34 to 36 ppm). Instead, make your own standard solutions and calibrate it to those. See here: http://www.reefkeeping.com/issues/2004-06/rhf/index.php> Looks nice but isn't 1.030 way to high? Thanks you guys and girls are great! <De nada and good luck, Sara M.> <I'm so sorry, in that last response I meant to type "34 to 36 ppt" NOT ppm. -Sara>

Specific Gravity Question  10/11/07 Hi Neale! I've been studying up on my next adventure - a marine tank (30-55 gallon) - namely with clownfish. I realize specific gravity measurement is important however in all the research I've done, I can't seem to locate HOW to measure specific gravity. There isn't a test kit for this is there? Relates to salinity doesn't it? <Hello Lisa. You need to use either an hydrometer or a refractometer. Or if you have the $$$, an electronic salinity meter. There's plenty about them here: http://www.wetwebmedia.com/spg_salinity.htm . I'd also add that a marine aquarium should be a mandatory purchase before going into marine fishkeeping. While you can sort of "pick things up as you go along" with freshwater fish, for marine fishkeeping you do need to hit the floor running. Bob Fenner's book 'Conscientious Marine Aquarist' is superb, and I'd say that even if the guy was a schmuck. As it is, Bob's a nice guy too. On the front page of the web site there's a deal right now for two of his books for a measly $30. Definitely worth considering, and given the price of even a single marine fish or coral, learning from someone like Bob will save you hundreds of dollars in the long run (not to mention the lives of those gorgeous sea creatures) -- http://www.wetwebmedia.com/daily_faqs3.htm? . Otherwise, browse, read the many articles on marine fishkeeping here at WWM.> On another note - is it possible for 2 year old livestock to grow when brought into a more spacious environment? (i.e. Mbuna, Plecos) <Yes and no. All fish grow constantly. This is actually standard for "lower" vertebrates. It's called non-determinate growth. It contrasts with birds and mammals, which grow to a certain size, and then stop. Anyway, what changes with fish is the rate of growth. This is normally fastest during the early stages of growth. In the case of a typical tropical fish, this will the first few months. Growth rate then slows down, to the point where after a few years growth is imperceptibly slow. The bottom line is that if a fish has been "stunted" in its first year or two, it is unlikely to ever reach the maximum size for that species, even if moved into ideal conditions. It's passed through the rapid growth phase, and all that's left is slow growth. In other words, yes, your Mbuna or Pleco will grow bigger in a new tank, but don't expect them to get *as big* as ones that had been in a bigger tank right from the start.> Thank you and I look forward to hearing from you! Lisa <Hope this helps, Neale>

Re: SG Problems... measure, refractometers   7/21/07 All I can say Bob is wow! Since sending you this e-mail I have been on a roller coaster ride of specific gravity instruments and their calibrations. I feel like I have learned a lot and thought I should report back to let you know what I came across (since this gets posted and all) <Do appreciate this> Okay, so in the beginning I put some straight RO on my refractometer and it registered as about a half an inch below where the scale even started. That's .5" below the 0 line on the instrument. This is what lead me to freak out and write you because once I dialed it up to 0 my salinity read 1.029 which had me near panic as at that time I thought it had likely been that high for months. Since then I began digging on the forums and found that with the decline in price on refractometers, the quality of the instruments has also been in decline. Basically the cheap ones (which I have) become unreliable the further the reading gets from the calibration point so it is much better to calibrate with a solution at 1.026 than it is to calibrate it with fresh RO at 0. Thanks to Randy Farley on Reef Central I found a homebrew calibration solution that was extremely simple to make, once I recalibrated my instrument using this solution, it turns out that my 1.029 water is actually at 1.022!!! and likely has been for all these months because of a bad refractometer calibration with straight RO. This 1.022 reading has since been triple checked against another refractometer, a conductivity meter, and a swing arm. Basically I wanted to write you back so that when this is posted, hopefully some people catch it and realize that they need to rethink how they are calibrating these instruments and not blindly trusting them like I did. I am now using my fresh saltwater for my auto top off to slowly bring the salinity up to 1.026 and feel as though I have been on a roller coaster ride thru a salinity nightmare. Anyhow, cheers to you guys and I really hope this helps at least one other person. Now that I have this behind me I am off to have a margarita.... hold the salt. <Heeee! Thanks Bush. BobF> Bush Williams PS, here are the links I was talking about: Calibration discussion: http://www.reefcentral.com/forums/showthread.php?s=&threadid=953605 Homemade Calibration Solutions: http://www.reefkeeping.com/issues/2004-06/rhf/index.php Hydrometer Inaccuracies 4/20/07 What is the best way to measure salinity? <A refractometer.> I bought a normal plastic Instant Ocean pendulum hydrometer but when I test my SG it will measure about .002-.003 difference when I take a water sample to my LFS. <These are pretty unreliable.> They use the exact same hydrometer I use. Do I need to calibrate it and if so how do I do it?  <They can't be calibrated.  Best to make sure that the water temperature is close as possible to what the hydrometer wants and there are no bubbles on the swing arm.  My advice is to invest in a decent refractometer, much more accurate and easy to use.> <Chris>

Refractometers hi guys:) <Hello...Jorie here> Quick question, I have 3 hydrometers and get a different reading off each and am fed up. I want a refractometer and don't mind paying the extra buck for quality.  I was wondering if you would feel comfortable with a refractometer from 1 one of the fallowing manufactures. Ocean Tech, Vital Sine,  Captive Purity. Which would you pick? Do you have a better choice perhaps? ocean tech, vital sine    http://www.drsfostersmith.com captive purity:   http://marinedepot.com    <I purchased the following refractometer about a year or so ago and absolutely love it.  Knock on wood, have had no problems whatsoever and would without hesitation recommend it to anyone in the aquarium hobby.  No need to pay the "big bucks" for all the bells and whistles unless you are a chemist, in my humble opinion! http://www.reefaquariumguide.com/sponsors/specials.php many thanks <You are most welcome.  Take care, Jorie>

Pinpoint Salinity Monitor Problems 2/18/06 Hi gang,  I've run a Pinpoint ph monitor on my reef system for several years. . . overall I'm pleased with it, although it has to be run with a 9 volt battery. . . two attempts at using the optional adapters provided by the manufacturer went astray -- I had a techno-geek friend test the second one after it shorted out my unit for a while. . . turns out the adapter was supplying 11.8 volts. . . way too high, and almost destroyed the monitor. The salesperson at my usual internet supply source confirmed the adapter unit is cheap/unreliable. <Indeed, the power supply problem is well known among Pinpoint users, and you would think they would supply a better quality unit, but alas... pinpoint monitors are made to be inexpensive hobbyist models.  Top of the line models cost much more.> On to my real point. . . a month ago, I ordered a Pinpoint salinity monitor. After calibrating the unit, I added it to my reef tank. Readings were WAY high and fluctuated wildly. I called the supplier. . . who was immediately aware of the 'problem'. Turns out the unit won't function properly in proximity to electrical current. . . e.g. my power compact reef lights. Since the only places available for mounting on my system are a crowded sump with several hundred watts of pumps being powered up (returns and downdraft skimmer pumps) and the top of my reef tank, there's nowhere I can run the relatively short (less than three foot) probe-and-cord to use the unit. I feel like I've effectively purchased a hundred-dollar-plus paperweight. As the last straw, the unit ate up a 9 volt alkaline battery in just a week's worth of use. The thing is, there's no disclaimer describing this problem on the website where I bought the thing. And the supply source wasn't willing to allow me to send back the unit to upgrade for another Pinpoint product. So 'caveat emptor' on this one. . . Chuck <It is a shame that this problem exists, and pinpoint should probably at least state in the instructions that the unit must be away from such interference, but again, these are entry level inexpensive products.  Your comments are appreciated and will be posted amount the FAQ's.  Best Regards.  AdamC.>

Refractometer Accuracy  1/20/06 Hi WWM, from snowy Nevada!! Hope there is better weather where you are! <Ah yes. A beautiful day in Georgia.> I have a refractometer that is approx. five years old. I recently had reason to question the accuracy of the meter, when someone else was testing the same sample water with another refractometer and received a completely different result than my refractometer.  Both refractometers had been calibrated. I purchased another refractometer as did someone else I know. We took all 3 refractometer's and calibrated them at the same time with the same water. Then took some sample aquarium water and tested it with all three. My five year old refractometer tested the water at 1.025, my new refractometer tested it at 1.025, and the third refractometer tested it at 1.022.  Is this a significant difference between meters that should be of concern? <Not really.> It seems the results should have been more similar, given that all three were calibrated in the same way, at the same time, with the same water. <Actually, most are only accurate within the certain temp. planned for their use. Probably what is the case here.> Another notable tid-bit, both of my refractometers have the same model number RHS-10ATC, and the third has a model number of ZGRS-10ATC. Could this just be the difference in models? <Yep. Probably meant for best results in certain temps.> None of the meters list any kind of accuracy reference. Which should we consider more accurate? <Try to find out the intended temp. for each. As long as you get consistent results from the one you are using, you should be fine.> I am very curious to hear your thoughts. <Oh, and I do have some curious ones!> Thank you very much and have a great night (hopefully in some tropical location)!! Jen Marshall <Hmm...Tropical? Well, there are some "strange birds" around here! - Josh> Hydrometer Readings  3/16/06 Hello there, <Hi Czarina - Tim answering your question today!> We have just set up a new 40 gallon aquarium. It currently contains nothing more than saltwater and live sand (with sump, bioballs, heater set to 77, pump etc). However, EVERY time we test the water with a hydrometer we get a completely different reading. Sometimes the needle hits the very top of the meter, then 3 seconds later the bottom, then somewhere in the middle (not necessarily in any particular order). We went and bought a second hydrometer to double check, and exactly the same thing is happening. We are taking these readings without leaving any time between them- so there is no room for any fluctuation. We have not added salt for a few days either. The guy in our LFS said there is no way this could happen. Also, it seems that when we put it under and hold it there, we get a higher reading it seems- than if we fill it and lift it out quickly. <It could be that you have bubbles stuck to the needle when you first fill it, causing it to jump high, then fall once the bubbles come off. Alternatively the needle could be getting slightly wedged at the bottom. In any case, fill the hydrometer to the recommended level - and ensure that your water is at the appropriate temperature for measurement (should be ca. 25C for most hydrometers but check the instructions supplied by the manufacturer). Then, depending on the model and whether or not possible, place the hydrometer on a level surface - e.g. a table top or similar. Tap the outside of the hydrometer (if it is plastic - not glass) with the tip of a pencil or pen a couple of times to dispel any bubbles in the hydrometer. Then allow the needle to stabilize and you should have an accurate measurement. I think the answer really is down to physics here!> I am completely and utterly confused as to why this is happening. This crazy fluctuations can't be possible <Exactly - they can't unless something on the needle is causing this such as an air bubble.>. Please advise us! Thank you for your time, <Always a pleasure!> Czarina.

Refractometer vs. Hydrometer question    11/27/06 Dear Crew, <Dear Eric, Mich here, hello to you on the first day of deer season.> I have been using a Red Sea Hydrometer for as long as I've been in the hobby which is about a year now. I decided to upgrade to a $60 Hand Held Refractometer w/ATC on sale for $30. <Good investment.>  I calibrated it this morning by setting the room temperature to approx. 68 degrees F. and a couple of drops of distilled water as directed. I then proceeded to test some buckets of pre-made salt water. To my astonishment every bucket was 1.028 SG. I went to my main tank and it was 1.0275, which is probably due to it being 78 degrees. My quarantine tank was the same. My hydrometer has always read 1.0235 consistently. I would do this 3 - 4 times in a row to ensure the result was consistent if not accurate. <As you know consistency and accuracy are two different measurements, both equally important.>  I thought it might be off by .001 but not .0035 - .004.  <Hydrometers are notoriously inaccurate.> Thankfully all the corals in the tank are doing fine. <Good.>  I only have two fish: a blue/green chromis and a Hippo tang who thinks he's a chromis. The tang has some scarred looking flesh which has been there since a week after I got him. Could this be from too high SG? <Could be may things.>  (it's a grayish color in patches and his tail has looked frayed forever. The cleaner shrimp enjoys picking at him when the fish lets him.) <Good.>  Is it possible that the Refractometer is also off? <Doubtful.>  Should I trust the refractometer <Yes.> and slowly drop back to about 1.025 or 1.024 over the next few weeks during water changes<Yes.>? I think I'll bring a sample of water to the LFS to double check before I do much of anything, <Good...if they use a refractometer.>  but I would like to hear what you have to say. <I would trust the refractometer, not the hydrometer.>  Is it normal for Hydrometers to read low? I've heard they can read high, but not low.  <Hydrometers can be inaccurate in any direction.> Thanks for your time - Eric <Welcome.>

Measuring Salt Content -- 10/25/06 Hi Eric, <<Hey Ken>> I finished the tank, filled it up, turned it on, and no leaks. <<Yea!>> What a relief. <<Indeed>> I want to test the SG in the tank.  I have a Blue Line ( I think) model RHS-10ATC refractometer.  I get a reading of 1.025 on this.  I also tried a Coralife "Deep Six" hydrometer and get between 1.021 and 1.022.  Does this seem normal to have this kind of range? <<Sure...the plastic box-type hydrometers aren't consider very accurate, but they are generally close/consistent enough for hobby use and are convenient for 'quick' checks, especially if you have something like a properly calibrated refractometer to use for reference.  Personally, I prefer to use an electronic meter for measuring salinity but what you have should be fine>> Thanks, Ken <<Always welcome.  EricR>>

Meniscus...Is It Up Or Down? -- 10/12/06 Dear Crew, <<Hello Eric>> Thanks for all the help you have given me in the past. <<Quite welcome>> I currently use a floating hydrometer/thermometer to measure the specific gravity in my tank and replacement water. <<Mmm, not very accurate...unless you have spent money for one of the better 'lab grade' hydrometers (they generally don't contain 'thermometers') as offered by Salifert, and pay attention to/have a conversion to allow for water temperature>> When using such an instrument, I have noticed that there is a reverse meniscus (shaped like a ' n ' instead of a ' u ') around it.  Do I read the specific gravity by checking the level of the water surface or where the reverse meniscus reaches?  I know usually a meniscus is read from its base but I have never dealt with a reverse meniscus, if that's what it is called. <<Hmm, describing the meniscus as either an 'n' or a 'u' is a matter of interpretation I think.  Regardless, read the hydrometer where the surface of the water just outside the meniscus would strike it>> Thanks, Eric <<Regards, EricR>>

And then there were none... substantial obeisance, hydrometer accuracy, troubleshooting SW illness...  - 09/14/06 Hi, You guys were a tremendous help to me last time, I went to Amazon and ordered anything with the name Fenner on it. (great books) A little background on the tank. I have a 50 gallon saltwater tank, 2 years old. pH normal, Nitrates normal, Nitrites normal, SG 1.022, temp.77-79, Nh3 negative. 30 Lbs live rock, and a 1.5 inch bed of live sand (infested with what I think are bristle worms, little orange things that are grey at the end). The lights run 10 hours a day, and I have a very healthy growth of bubble algae and the like. ( the tang loved it ) In the last three weeks I have lost,,,,  A yellow tang, 2 clowns, 5 assorted damsels, a mandarin (sob!) <Yikes>   and a citron goby. I have introduced an anemone, and a feather about two weeks ago, but they seem fine. <... likely the Anemone is involved, related to your fish losses here> My crabs and snails are all fine, but I tried to put in cleaner shrimp to help stem the massacre, and they died about 20 minutes after being put into the tank. (do I need to recalibrate my hydrometer?) <Doubtful... but I would "check" it against a known-to-be more accurate device... and raise your spg to 1.025> The fish were coated with a grayish mucus especially noticeable on the eyes, I have cultured the water <Neat!> and found Gram negative bacteria such as, Proteus, and Pseudomonas. <Very common "similar" microbial involvement in all marine waters> The fungal culture will take a bit longer to grow. I rarely do water changes, but the tank evaporates about 2-3 gallons a week that I refill with RO water. I have a canister filter that the LFS guy says not to touch "if your water is so clear, and the parameters are so good, don't even clean it, you will kill the bacteria that  live there", so I added another smaller canister filter. <Mmm... am not a big fan of this technology for the vast majority of marine systems. See WWM re> I don't know what to do, this has happened at the end of last summer as well, and I simply left the tank empty for 6 weeks to crash any parasites that were there, and then added fish and they were fine, but this time I don't think it was parasites. HELP PLEASE!! many thanks, josh <Well... Please read here: http://www.wetwebmedia.com/anemcompfaqs.htm and here: http://www.wetwebmedia.com/marine/setup/filtration/marineFiltr.htm and the linked files above.... And consider adding a protein skimmer if you don't have one, better filtration period, trading in the Anemone... Bob Fenner> Hydrometers Hi Bob- Which hydrometer would you recommend. Aquarium Systems Sea Test or  Coralife Deep Six. Also should I hard plumb my w/d with the hang on overflow or use the flex hoses? Thanks Tom >> Six of one, half a dozen... they're about the same... more important in terms of accuracy and care of these measurers of specific gravity is learning how to use them... tapping their boxes to rid air bubbles, rinsing them, and storing them carefully (a zillion dollars worth will be broken today!) especially. These plastic "box type" hydrometers are better than glass types though... and accurate enough for commercial and hobby uses. Bob Fenner 

Hi Bob, I have one other question for you. Is their a formula to convert temp, salinity, to specific gravity? pH ORP Temp Cond. Sal. 8.13 323 80.2 58700 38.49 8.19 317 79.7 59500 39.06 8.11 320 80.1 60100 39.49 8.14 319 81.4 60100 39.48 8.27 319 80.9 59800 39.27 8.12 316 81.4 60800 39.98 8.05 316 79.8 60200 39.56 8.03 315 79.8 60200 39.56 8.01 316 80.3 60800 39.99 nitrate 0, nitrite0, Amm very little, all most not detected Alk 5 meg Cal 425 phos .1 dKH 10 Thanks again, Larry >> Hmm, I'd rather offer a table rather than a formula here... And a warning to not get overly excited about this aspect of water quality... best to "return your water to center, or thereabouts" via water changes on a regular basis, as more than salinity is at play in figuring to or from relative density (specific gravity) or seawater. Assuming a constant salinity of 34ppt, varying temperature, the spg should read: Temp.F Temp C Spec. Gravity 86 30 1.021 84 29 1.021 82 28 1.022 81 27 1.022 79 26 1.022 77 25 1.023 75 24 1.023 73 23 1.023 72 22 1.023 70 21 1.024 Bob Fenner

My fish are sick Thank you for answering my questions. I guess in my efforts to give you all the information you needed, I overlooked one of the most important things. I do have some test kits I use. The ph level is and has been 8.0 the entire time I have had the aquarium. My nitrites are and have been 0. There aren't traces of ammonia. The only thing that has changed during the time I have had everything are the nitrates. They started at 0 and after about 2 months increased to 20, after about 3 weeks increased to 40, and have stayed at 40 for the last few weeks. How do I lower my specific gravity? How do I measure it? <With a tool called a hydrometer... and it can be lowered by removing some of the seawater and replacing it with just fresh... Maybe take a look at the article on specific gravity and salinity posted at www.wetwebmedia.com> I did do a partial water change a week before everything started and I haven't been able to find a new filter for my Fluval system (small town pet shop hazard, I will end up having to order it from the internet), so I just rinsed the sponge filter out. We have well water and haven't had any problems with it, but could that have caused everything to go out of kilter? <Yes, possibly... you might consider having your water checked for you and your aquarium use... and do what I do... get a small Reverse Osmosis unit for drinking, cooking and pet-fish use> I did turn up the aeration system after I added the triple sulfa since it seemed to create a film on top of the water, and have left it up since then. Jazz did die sometime last night, as I had feared he would. He was my favorite. Thanks for the link to WetWebMedia, there is a wealth of information I will take time to read through. Also, if there is a good "Salt Water Aquariums & Fish for Dummy's" or maybe a "Guide for Panicky Pet Owners" book you recommend, I would like to have some references on hand with color pictures if possible. Again thank you for your time and advice. Sincerely, Millie Opela >> Do take a look at some kanucklehead's version of the way marine aquariums are supposed to be titled "The Conscientious Marine Aquarist"... a good read, and full of nice pix. Bob "immodest tonight" Fenner

Re: Chocolate Chip Starfish ... I removed him last night when I saw absolutely no more activity from his little feet. Someone suggested it might have been a change in salinity, but salinity has been fairly constant between 1.022 and 1.023... I'm using an Aquarium Systems SeaTest Plastic Hydrometer for warm water aquariums. Can you recommend something better? or is this okay? water temp around 78F >> >> This is a fine (accurate, precise) brand, type of hydrometer. It is what I have used for many years. Bob Fenner

Water Density Measurements... First off I just want to say what a great book Conscientious Marine Aquarist is. It has been invaluable to me and its the first book I recommend to anyone who's asked me about getting into this hobby. I have a question about reading my hydrometer. I feel kind of silly, but none of the tables I've seen for temperature adjustments have made sense to me. I have a hydrometer which is calibrated at 78F. For my water at 82F, it gives a reading of 1.024. What is the true specific gravity/salinity of my water? And more importantly, how do I adjust the reading for those 4 degrees? Thanks a lot. >> Hmm, the "true" specific gravity? In all likelihood it is very near the 1.024 value that your hydrometer is calibrated to... and I wouldn't obsess about the absolute spg at any length... there are a couple of variables, artifacts if you will, that influence hydrometers readings on the basis of temperature... (compression and gas solubility issues...) but these are far less important (as is real near-seawater density questions) as doing your best to keep spg about constant... by pre-mixing your synthetic water, topping off occasionally with fresh... Bob Fenner, who thanks you.

Trusting Hydrometer Hello- First off thanks for your devotion to this hobby.  <To hobbyists and the planet> I recently moved and had to make the change from well water to city water. Upon setting up the tank (75 g. with sump, skimmer etc.) I had major problems. Lost all shrimp, inverts etc. the first two days. All that is left is a green Chromis and a Sailfin tang. At first I thought the problem was lack of dissolved oxygen from disturbing the DSB, (and maybe part of it was) but after reading your articles and FAQ's I was concerned about my salinity, it seemed as though I was adding more salt to my water than I normally did to get my "bobber" hydrometer to read 1.023. I went out and bought a new Hydrometer swing arm). To my horror it registered off the chart, above 1.030. <Yikes> I felt/feel horrible. I exchanged a few gallons of water for fresh water, but I don't know what the s.g is at or how much to change it a day, I know you say not more than .001/24 hr, but this is hard if I am off the scale. <Still, better to do gradually at this point...> I am having trouble believing my instruments, this is a bad feeling. Should I invest in "refractometric means" of measuring salinity. Sorry this is so long, please advise. <The refractometer is a nice tool to have/use, and valuable for research, having lots invested... but a decent "regular" calibrated hydrometer (and/or one to check it against that you can trust) should yield sufficiently accurate and precise measures. Sorry to read of your losses. Bob Fenner> Thanks, John Gray

Refractometer I have a Refractometer and a Hydrometer (deep six) which do you consider a better choice? <Mmm, better choice?... Well, the refractometer is decidedly a "better choice" for accuracy and precision of measure of actual salinity... And the hydrometer is the "better choice" for ease of use (though not as accurate or precise) in measuring density/specific gravity. I would likely use the latter for most daily, occasional use.> I used 5% NaCl solution to calibrate the refractometer and my salinity of my tank, which read 1.023 with the Hydrometer, read 1.017. I cannot convince myself which is better. <Mmmm, you may be becoming "too involved" here (like myself, we would likely lose on Jeopardy (tm) due to too much considering, thinking about the "real" answers... when a simple, general sense of what is involved is all that is necessary). Please read here: http://www.wetwebmedia.com/spg_salinity.htm and the FAQs beyond. Be chatting, Bob Fenner> Alex

Hydrometer (addition of salt to koi pond) I need your help in testing how much salt I need to add to my pond. I have done a partial water change and I need to know how much salt I can add. I bought a "SeaTest Hydrometer" and I need to know if is possible to test *my Koi pond water with this. If so, what should the reading be on the meter for a pond with plants. Thanks for your help. <Mmm, not really useful/practical to use a hydrometer for salt additions to ponds... the small amounts of salts in such applications are not easily, precisely, accurately measured by such a tool. The long and short of what I sense you want to know is "just how much" salt to add... as in per the volume of your system, end results of the addition. I would guesstimate how big (gallons... as in about 7.5 gallons per cubic foot... or by filling with a watch... and figuring how many gallons per minute in filling a "pickle bucket" of determinate volume), and add (incrementally, over a period of days), about a pound per one hundred gallons.  Much more to say/state... if you were interested in "how much" real salt you have already, and to augment you might look into a conductivity meter (rather than density measure with a hydrometer) or other "marine aquarium" salts measuring gear... Do take care in how much, how soon you add salt to your system... as the rapid die off of algae, sometimes beneficial nitrifiers, increased osmotic pressure... can be too stressful to your livestock (fishes, invertebrates and plants)...  And more to the point (at least mine) is "what do you hope to accomplish" by salt addition? If any of this is incomplete, not clear to you, let's keep discussing ahead of application. Bob Fenner>

Hydrometer accuracy? Okay. I have heard that the floating hydrometer that I have (Marine Enterprises about 13" long, calibrated at 75 degrees), although supposedly a very good one, may not be accurate. I also wanted to be able to test from my tank a little easier. So, I went out and got a SeaTest Hydrometer by Aquarium Systems. It immediately jumped to 1.0295!!!! My floater always reads 1.024-1.025. And, yes, I did knock the bubbles off of the SeaTest float prior to reading the results. Which one is correct? <Very likely the floating unit is closer to what is real> What should I rely on? <Please read here: http://www.wetwebmedia.com/spg_salinity.htm> My calcium test is not working as advertised and I am wondering if the salinity might be a problem. Carbonate hardness looks okay at 110. Linda Swenberg <Good lessons to be learned here re accuracy, precision, the "nature" of tests, testing. Bob Fenner>

Plastic Hydrometers Hi there, Currently, I'm having two brands of hydrometer. One is from aquarium systems and the other one is from marine enterprise inc. They're both giving me different set of readings. Which one should I trust ? What would be the effects be (inhabitants consist of fishes and corals) if the SG level is high (let's say it's above 1.026). Thanks. <Plastic hydrometers are known for this. I would use the one that reads zero with RO water.  Some people average between the two. As long as the new water matches the old water and the SG is between 1.023 and 1.028 (a bit high) you're alright.  Craig>

Plastic vs. glass hydrometer Anthony, Again, I can't tell thank you enough for you advice.   <truly a pleasure> I thought I'd share with you some news today.  You mentioned in my original question that there must be a simple answer...I think you're correct.  On a whim, I took a sample of tank water to work with me (I'm a veterinarian), and checked the salinity.  My refractometer read 1.017!! I couldn't believe it.  My hydrometer at home is telling me 1.024.   <heehee... piece of crap plastic hydrometers... I don't know why they even make them. Don't get me wrong... the cheap glass hydrometers can be equally inaccurate... but they don't stray. They are consistent and can be trusted after a single comparison to a reference point like a refractometer for those that cannot afford one. Plastic hydrometers can be corrupted in so many ways- junk.> I was so dumbstruck, I checked it twice more on separate refractometer.  One read 1.016 one read 1.017. >Please tell me this is the likely cause of my problems.   <hard to say... but very stressful indeed. And the difference in success between species was the difference in tolerances perhaps> If this is it...I'm not sure whether to laugh or cry.  I have closed off my top off, and assume that it is best to allow natural evaporation to gradually return the salinity back to normal. <its not that scary low that you need to add salt. I'll agree> Jeff <best regards, Anthony>

Refractometer 2/28/03 I recently purchased a refractometer designed for salinity measurement.  After calibration, I noticed that it was way off from what I thought I had in my tank.  Using a plastic SG meter, I measured 1.024; with the refractometer, it was 1.029.  I keep the plastic device clean and free of build-up, particularly on the pointer.   <very good... and for all of the daily FAQ readers: it is a necessary habit to give your plastic/glass hydrometer a rinse with distilled or deionized water after every use to prevent mineral build-ups that will skew readings in time> With another, brand new plastic meter (from another company), I also get 1.024.  I read up on refractometry, especially regarding its use in determining the SG of urine.   <I hope there isn't a seawater v. urine analogy coming <G>> In that context, I read that the accuracy can be affected by protein.   <dammit... ;) > Reasoning that even with a good skimmer there is dissolved protein in the tank water, I thought that this might account for the discrepancy.   <ahhh...no> After all, a display tank will have more dissolved in it than just sodium chloride.   <the amount of proteins/incidentals in urine is... ahhh... very different from habitable seawater. (Although I have seen some aquariums in my travels that were so neglected that it looked like the fish were swimming in piss). Refractometers are used everyday in seawater by science and aquarists alike in the tens of thousands of unit numbers. The problem with your refractometer is either simply that it is a defective unit... or, more likely, that it is a hobby grade unit or a handheld unit. Handhelds are slightly effected (more so) by temperature than table tops (although the difference would not be so great to explain the .005 dif you noticed). Furthermore... those $50-100 hobby models are dubious if not junk too often. For that kind of money, you could get 2 or 3 top shelf glass hydrometers that are much more accurate and never need calibrated.> I would be curious about your experiences with refractometry to determine SG/salinity. thanks tom <if you do have a lab grade or table top refractometer, then I am honestly at a loss to explain the discrepancy short of defect. Refractometers are nifty toys... but really not necessary. Use your plastic hydrometer for convenience and safety (no matter how many times I call them junk <G>) and have a decent $20-50  professional glass hydrometer on hand to spot check the plastics periodically with. Best regards, Anthony> Store owner looking to measure saltiness I own a local pond retail store and I need a tool to accurately measure salt in my koi pond do you sell this tool? <Mmm, no. We don't actually sell anything but the books we produce, but you can likely buy a refractometer (most accurate, precise tool... from most any wholesaler that carries marine gear or etailers. A bunch are listed here: http://www.wetwebmedia.com/marlinks.htm Bob Fenner>

Refractometer Problems Hi guys, just picked up one of these units... <Phil here wishing he had a refractometer.> the unit says it is calibrated to 0 when shipped, I used a couple of drops of bottled spring water and got a SG reading of 1.004, then with tap, same reading, then with tap I boiled for a few minutes, same reading, then tested tank water 1.027, tested tank water with old hydrometer 1.022, should I think of the refractometer as accurate (not against the hydrometer as I know the refractometer is more accurate, but wondering why the reading of SG with the plain water used), or should I adjust that 1.004 reading down to 0 and then consider my tank readings to then be accurate, I already adjusted it down to 0 and now am getting a tank water reading of about 1.024.....Thanks again... <Hmmm... I would check with the maker of the unit.  It could be that the refractometer was damaged in transit to your home.  But if I was in your shoes I would just adjust the reading as you have done.  Hope this helps!  Phil, who is going to check his SG using the "plastic box"... LOL>

Refractometer Problems Try testing your refractometer with RO or distilled water. "Spring" water comes from the ground and has mineral content read by the refractometer.  Same with tap and boiled water would/should actually be a bit higher still as the actual water evaporates from boiling.  Try it with pure water. 

Hydrometer Hi guys, looking forward to the new book, it will hit Canada I hope?..... <indeed... we have several distributors of it waiting for it in CANADA <G>> I have a question, I use an old "deep 6" swing arm hydrometer, I always mix my salt so that it is the same SG as my tank (.024),now I know about these being calibrated at a temp of 75f,my FOWLR tank is always at 80-82f.....am I still at .024?,any idea what I am really at?, does it matter as long as consistent?.....Thanks again.... <mostly the latter (consistency). Published charts of the temp correction of salinity over a wide range reveal very little appreciable change. Not even noticeable between 75 and 80 F on a plastic hydrometer. No worries. Anthony>

Hydrometers Hi Bob, <Hello Tyler> I have read your articles regarding the care of one's salinity and I agree that testing in a separate container helps prevent breakage; however, I recently bought a new glass hydrometer made by Living Sea, and it is labeled "Marine Enterprises SALTWATER HYDROMETER/THERMOMETER specific gravity 75°F".  The problem that I have is that the chart that I have for converting specific gravity to salinity was designed for an instrument calibrated to read 0 in freshwater at 60°F---I'm presuming that the instrument I now have is set to read 0 in freshwater at 75°F---how might I compensate for this dichotomy (or where might I find a conversion chart for 75°F hydrometers)?  Or does the difference in index temperatures matter at all? <Does matter, or should I state "it can". Likely at the tank temperature you're going to be at this device will register a couple of thousands (0.002) low on actual specific gravity. You could do an actual check (with water of calibrated temperature and raising it to tank temperature) to see what the difference actually is. I would do this and make the mental adjustment (that is, that a measure of 1.025 is likely to read about 1.027). The usual note here: More important than the actual specific gravity is its constancy and even more important the composition of the media...> Thanks for your advice, Tyler Reynolds <Welcome. Bob Fenner>

Re: Hydrometers So the difference will be arithmetic, not geometric or parabolic? Or is it simply too similar to warrant distinction within the boundaries of 1.020 and 1.025? <The latter. The relationship is not arithmetic, but differential, however "close enough" w/in the "nuts and bolts" of actual aquarium practices to be so here. Bob Fenner> Thanks, Tyler

Measuring Salinity... the long way around the barn 6/23/03 I have a TDS meter and I was wondering if its reading could be used as a measure of salinity which could be converted to specific gravity. <really not necessary/ideal> I got a link to an interesting site in the forums, but I can't get a good correlation between what my tank reads on the TDS meter and its specific gravity. I was hoping you could enlighten me. <use you instruments instructions or a good conversion web site to work in reverse. A good average for seawater is 32ppm (about 1.024 SG)> My understanding is that a TDS meter basically measures the conductivity of water (so many miliSiemens/cm) and then transforms that reading to parts per million. <yes... but it can be influenced by other solids/additives you put in the water> Conductivity is related to the solids dissolved and should increase as salinity increases. Can such a TDS ppm reading be converted to salinity? <you are an engineer aren't you <G>?> If it does, what is the salinity or TDS reading that corresponds to say 1.025 specific gravity at 80 C? <your father was an engineer> TDS meters are fairly inexpensive nowadays. <ahhh... a glass hydrometer (not plastic!) is more reliable, does not need calibrated... and costs about $6> It seems they are fairly accurate and it would be nice if they could be used for more than gauging the health of the RO membrane in an RO system. <OK... but over thought/unnecessary IMO> Thanks for your help. Henry <best regards, Anthony>

- Conductivity & Salinity - Quick question: I have recently acquired a Milwaukee conductivity monitor. I want to use this to monitor the salinity of my tanks, but I cannot find a table for converting conductivity to salinity at a given temperature. I have looked all over the Internet. Any idea where I can find a table or graph? <Well... my only tool available here was the Net, so I used Google to find this: http://www.radiometer-analytical.com/all_resource_centre.asp?code=112&s=go I think you might find the information you need there, but you should also contact Milwaukee Instruments.> Thanks, Steve. <Cheers, J -- >

Hypo-salinity and Hydrometer Accuracy Hello Bob, Steven, Jason, and the rest of the crew, This is my first question for you so I want to say that your site is wonderful and has provided me with a wealth of information, which has allowed me to be much more successful at this hobby than I could have ever thought.  Without this site, I would likely be one of those in and out reefers who drops the hobby after a year, now I'm hooked for life.  I'll keep this short, no story behind why I'm asking.  I am having a problem with hypo-salinity treatment.  I believe that my hydrometer readings are off.  I have three: a Deep Six, a Sea Test box-style, and a floating glass (with a built in thermometer and made for aquarium, rather than lab use).  All three read differently!  So I chose to trust the floater because I have read the others can drift over time and both are not brand new.  I have conversion charts galore, but they are worthless if you don't know the calibration temp of the hydrometer or if the hydrometer is not accurate anymore.  I believe I am failing to maintain a proper 1.009-1.010 SG and that my true SG is more like 1.012-1.013 (this would be true if the floater were calibrated at 60F).  After three weeks in hypo and several FWD's my fish are still showing spots and scratching a bit. >>Do know that there are documented subspecies of Cryptocaryon irritans that do quite well in low salinity environments. >Questions:  Is there any way that I can mix a test sample of water with a controlled amount of salt (I have Instant Ocean brand) at a specific temperature in order to test the accuracy of each hydrometer?   >>Yes.  I would first calibrate with distilled water. >Is there any other way for me to be sure that I am at the proper SG level?   >>Yes!  Invest in a good quality refractometer! >Even if I am now at 1.013, is it possible for the ich to be so virulent as to survive that, and continue to re-infect my fish?   >>Yes.  This would necessitate the utilization of copper or formalin treatments. >Can the spots I see be bacterial infection from the parasites bursting out during FWD's? >>Yes, but not so likely as to keep the same appearance of the ich (at least not in my own experience, secondary bacterial infections really LOOK like infections). >With sincere gratitude, Manny >>Do a search on Terry Bartelme, he's written quite a bit on ich, treatments, prevention, etc.  Then, I would search the reefs.org library, as well as Advanced Aquarist for the same.  I've only recently learned of the ich subspecies, both instances mentioned by folks working in Hawai'i.  Hope this helps!  Marina

- Specific Gravity & Temperature - Hi, Hope all is doing well there.  I have a 75 gallon F/O tank.  My fish are:  2 percula clowns, 2 lemon butterflies and 1 coral beauty angel. Please tell me what you recommend for tank temperature and specific gravity.  I have been keeping the tank at 76 degrees and the specific gravity at 1.021.  Thank you, James <James, I'd shoot for 1.025 for salinity - it's what the ocean is typically at. As for temperature, you could go a little higher, but there's nothing wrong with 76F - 76-78F is ideal. Cheers, J -- >

Specific Gravity Specifics! Hi Scott <Hi there!> My salt water aquarium has been running now for a week now since setup. I mentioned to you that I am trying to sort my heat issues out and in the last week my tank has been running close to 80 deg Fahrenheit. <Not too bad...> What I need to know is does the temperature affect the salinity reading? <It can> I hope this is not the case as I have my salinity measuring 1.022 which is perfect right? <Ya' know what? I used to be a believer in lower specific gravities for a lot of reasons, but after much experimenting, I've concluded that natural ocean specific gravity (generally 1.024-1.026) is the way to go...I shoot for 1.025> I should have the fans in soon and also gonna be doing one or two mod.s to the hood of my tank so I can get more air to circulate. <Excellent! That can make a difference in the heat buildup...> Thanks Again Regards, Ziad Limbada <My pleasure! Stay cool! Scott F>

How Do I Use This Thing?  Float Hydrometer Primer >Hi, I'm setting up a 110g saltwater tank. >>Cool. >I was given a glass Saltwater Hydrometer (by Marine Enterprises), approx 12 inches in height. The problem is: I have no idea how to use it. No instructions were given with it. >>No problem.  They don't come with instructions for the most part.  It's a float hydrometer, and it's really easy to use. >Is there a website with step by step instructions on how to use the meter properly. This meter has what looks like little pellet balls and some type of wax in the bottom of it.   Thanks for your response, America >>Easy-peasy, America, no website needed.  What you will need is something to put some of the water you wish to measure the salinity of into that is TALL enough to let the hydrometer float freely.  My favorite is a device I make myself out of clear plastic tubing (you can get this at the fish shop) that is capped and sealed on one end (needs to be watertight).  About 1" in diameter is fine.  Then, you fill it with the water, drop in the hydrometer, and you want to look at the lines on the really skinny part.  There will be one fat one that's pure water, a salinity or specific gravity of 1.000 - that's pure water.  (Btw, don't measure especially cold or warm water, make sure it's room or tank temperature - between 73F-82F - as this WILL affect the proper measurement).  So, the hydrometer should float so that the lines BELOW this 1.000 mark are what hit the top of the water - this measurement is your specific gravity. It is at this point that I MUST urge you to start buying books, because (and please don't take this as an insult, we all start from a position of no knowledge), if you don't know how to use the simplest hydrometer made, then I fear there is much other invaluable information you don't have as well.  There are MANY excellent beginner books, Bob Fenner's "The Conscientious Marine Aquarist" is one, and you may wish to search for books by Martin Moe, Jr., C.W. Emmens.. aw heck, to http://www.reefs.org/library/reading/  and http://www.reefs.org/library/reading/beginner/beginner.html and search the database for books on saltwater for beginners.  Also, you will find (if you have regular internet access) that the talk forums for our site and reefs.org are EXCELLENT for quick information.  Our sister site is http://www.wetwebfotos.com/talk - know that many of the folks who answer questions here are "on duty" there.  If you do join, let 'em know that Seamaiden/Sea Maiden sent ya.  ;)  All my best, and Happy Holidays.  Marina

Hydrometer trouble - 12/18/03 Hi crew, I've been visiting and reading the FAQ everyday for a year now and you have helped me many times. <great to hear> Thanks for all your hard work it is truly appreciated. <tanks a million, bud>  A couple of quick questions, yesterday I decided to do some maintenance on my sump/fuge which is a 30 gal that feeds my 55 gal reef.  My SG has always been 1.024 using a floating glass hydrometer. <very well> I replaced most of the sumps 30 gal with premixed saltwater, problem is after completing the water change I broke my glass hydrometer. <DOH!> I went to the LFS and bought the plastic "box" version, did a read, 1.030 (as you always say YIKES!). <Don't trust it> I want to start replacing 2 gallons per day with freshwater to get SG back to 1.025 area. <Woah!!! Get a second or third opinion. Try either another glass hydrometer or if you know someone with a refractometer>  I understand that consistency is more important than accuracy, however should I trust the "box" or get a new glass hydrometer for calibration purposes? <New glass and another opinion>  I don't want to over react based on a possible faulty reading from the "box". <agreed!!> Also if the 1.030 is accurate, based on 85 gal total water volume, minus 100# LR, do you think 2 gallons per day a reasonable starting point for moving SG .001 per day? <That should be about right you could do a little more>  I've read of similar events in the FAQ's, just looking for a "mental" calibration. <get a second or third opinion if possible> As always thank you for your kind response. <My pleasure. ~Paul> Mike

Hydrometer Recommendation Hi All, <howdy> I am looking for a recommendation for a Hydrometer / Refractometer for measuring specific gravity.   <your best bet/value IMO... a glass hydrometer (do spend the extra and get one of the $20-30 units). These are arguably more reliable then even the handheld refractometers> I have the usual cheap one, but am told I should get a better one for hypo salinity treatments in QT tanks, etc.  If you have a link of one or can comment on the refractometers on marine depot (around $70), I would greatly appreciate it. Thanks, Andy <the cheap hobby grade handheld refractometers are good... but not as great as one sometimes here's about. Reliability in refractometers is to be found in table mount models (not influenced easily by temp) and on lab grade equipment. Refractometers that are made cheaply overseas (imported $15-25 and retailing $50-80) are not always as good as you might hope for them to be. Whichever you choose... simply avoid using the plastic hydrometers as a primary. Anthony>

Hydrometer Recommendation 2/3/04 Thanks, do you have a link where I can find an acceptable glass one (perhaps a brand name)?  I checked marine depot and a couple other places online and can only find the cheap hobby ones.   <no easy link here I can think of. I have used the high end hobby ones like Marine Enterprises brand with satisfaction FWIW. Anthony>

Refractometers 2/14/04 Quick question regarding refractometer.  Love this site!   <thanks kindly> I have a plastic hydrometer, which from reading on this site is not the best of readers, nor was I aware of the cleaning.   <correct... they are not that reliable or durable. A quality glass hydrometer is actually the best all around. Do keep both and compare the glass as needed> Which I am doing now....  I work in a lab, we have a refractometer which is checked daily with two controls.  One is DI water, reading should be 1.000 +/- 0.2, the other is 8% NaCl with a reading of 1.032 +/- 0.3.  The controls are at room temp.  I see calibration is mentioned, and I note on the refractometer that is calibrated every 6 months.  Now can I use this for my salinity checks?   <certainly> I brought a sample in before and it was quite a bit lower than my reading with the plastic one. <not surprising... some of those plastic hydrometers are easily corrupted (dried minerals from poor cleaning, dropping/jarring... and QC> This refractometer is a Reichert TS meter. If I bring in a sample from home, the temp on that sample is quite lower than my tank ... my tank is at 78. Will this make a difference in reading?   <the diff is small but please do simply heat the sample up. Use a water bath to float your sample in (its container/baggie) to warm up> This is a lab refractometer (I work Hemo and Chemistry) we use this refractometer for urines, though usually this is only used after a problem with our automated instrument.  So can I use this, does the temp make a difference.   <yes... but there are published scales for compensation... mere fractions of a difference though> When I do any testing on my aged salt mix (to replace weekly changes) I always have the temp matching the tank. <very important...yes> Thanks so much Kris <best regards, Anthony>

Salt concentration links as requested   2/2/06 Hi again, Thanks for your quick response to my questions. I found these two sites helpful in determining salt concentrations. They both relate ppt (parts per thousand) and ppm (part per million) to percentage readings. The first of these is very reputable and most helpful, and deals most directly with my original question about ich and salt concentrations. The second deals with koi pond chemistry but was helpful nonetheless. http://edis.ifas.ufl.edu/VM007 http://www.koiclubsandiego.org/SUBcategory.php?categoryKey=3&subCategoryKey=19&subCategoryName= Salinity&PHPSESSID=f7a64e77d6b7b88b6b21f06b6b31ffb3 Thanks again Phil <Ah, thank you for this. Will post/share. Bob Fenner>

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