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FAQs about Marine Aquarium Heating, Measure/Thermometers

Related Articles: Heater Impressions (Reviews) by Steven Pro, Marine System HeatingColdwater SystemsControllers

Related FAQs: Heating 1, Heating 2, Heating 3, & FAQs on: Rationale, Heating Methods/Gear, Heat Controllers (Fans et al.), Heating Troubleshooting/Repairs, Makes/Models by Manufacturer, & Chillers, & FAQs on: Fans For Cooling, Chiller Rationale/Use, Selection, DIY, Installation, Maintenance, Troubleshooting, & Water Temperature

Best to have at least two thermometers... placed in different areas... These devices can be notoriously inaccurate AND imprecise. Stick-ons are okay... as long as you realize the insulating properties of what they're stuck on...

Accurate Thermometers 1/31/13
<Hi Judy, Rick this time. I have a physics background so I thought I'd field this for Bob.>
The fish are eating again and are acting normal. The temp was down lower than normal, in the mid seventies. I put two 100 watt heaters in instead of the 150 watt alone, now the temp is 81. I notice that the cheap floating thermometer with the suction cup always reads lower than the digital one with the wire hanging in the water. I noticed that they sell floating digital thermometers. I wonder which has the best accuracy? Thank you
<Regarding thermometers, you have two factors in play here, precision and accuracy. Precision has to do with repeatability. For any given temperature, the thermometer should produce the same (not necessarily correct) result.  Accuracy has to do with the distribution of repeated measurements averaging to the correct answer (but possibly with a wide variation).
Most of the thermometers for aquarium use are dirt cheap and mention nothing about how reliable the measurements are.  So, regardless of what kind of instrument used, there is always some question about how reliable the reading is.
Consider that I have used a thermometer where I could physically move the alcohol tube up and down the scale by as much as five degrees, and that doesn't lend much comfort to the accuracy of that particular thermometer.
In this case, it was an outdoor patio type thermometer, but it does demonstrate how careful we must be in never questioning the results from any given instrument.
That said, Take a look at the package or manual for the digital thermometer and see if it lists a specification.  Digital thermometers should be both more precise and accurate (see above) in general, and if the specifications are good enough to give you a comfort level, you can use it to calibrate the other thermometers--in other words, determine how far off the mark the alcohol thermometers are from the digital results. Once you know that, you can add or subtract the few degrees to what you read to get the correct value. And, you can use several of the cheap thermometers and use them with a lot more confidence.
Complicating all of this is the fact that your tank will have slightly different temperatures in different regions. In general, near the heater(s) will obviously be warmest, and farthest from the heater will be coolest. 
It's best to find the coolest spot and monitor that location as well as somewhere close to the heater so you understand exactly what is happening inside the tank thermally.
I know that's a long answer to a fairly brief question, but there is a lot in the question.  Hope that helps. - Rick>

3 years of inaccurate temperature  12/30/07 Hello Bob and crew! I have been reading your books and website for 3 years. I felt It was finally time to personally thank you guys (and gals) for the unimaginable number of time you have led me through questions about reef keeping. I would really like to send a donation, how would this be possible? <Mmm, the Amazon "begging bowl" at the bottom of most pages is the simplest vehicle> Also the only question I have today is what the best simple digital temperature gauge. I recently bought a Hanna ph/temp pen to help with Kalkwasser dosing and proved my Corallife digital thermometer was off by 3.9 degrees F. I cannot BELIEVE that. You get what you pay for! I am so happy with products you have recommended in the past. ( euro-reef, Iwaki, Tunze, etc.) 3 years at 78 degrees. Wait make that 81.9 degrees! UGGHHHH. I fell lucky the reef is as good as it is! Thanks again for your worthy cause! Jayson Dufresne <Thank you for this input... will post/share. I DO encourage you to send this unit and note to Corallife. Bob Fenner>

Broken Thermometer 3/29/07 Hey WetWeb Media, <Hi again.> Real quick question I was mixing water today and had one of those floating thermometers made of glass in the trashcan and when I poured the water in it broke the thermometer. Should I throw out the water and the trash can. <Yes to the water, no to the can, just rinse it out very well.> Because I think the thermometer had mercury in it.  <Unlikely, most now are alcohol based.> I still have the heater and power head. Should I even throw those out? <Nope, just rinse well.> Or am I being crazy. <Maybe a little crazy.>  Please write back as soon as you can. Thanks Jeff <Chris>

Laboratory Grade Thermometers   7/25/06 I'd like to tout a product that has been unwisely ignored by many aquarists. <<Okay>> This product is a liquid-filled thermometer that is accurate enough to calibrate a thermostat or temperature controller. <<Yes indeed, a "laboratory" grade thermometer can be a great help for determining the accuracy/degree of deflection of similar "hobbyist" grade devices>> I purchased two such thermometers last year and they are indispensable.  Most thermostats and temperature controllers must be periodically checked and calibrated. <<A good practice, yes>> The A-20 mercury thermometer that I purchased early last year from " http://www.sealifesupply.com/" is ideal for this purpose.  Its range, 66 to 80 degrees Fahrenheit, makes it ideal for aquarium use.  It is a foot in length with markings every 0.2 degrees.  Using this thermometer, I discovered that my digital controller, which displays temperature to 3 digits, had drifted by 1.5 degrees Fahrenheit.  I also discovered that some of my analog heater thermostats were off by almost 5 degrees.  Later, during a trip to NY, I purchased the nearly identical T-4900/S80F1 thermometer from "http://www.millerweber.com/".  The T-4900 is filled with a non-toxic blue liquid and is especially easy to read. <<Mmm...something coming of increasing importance to my aging eyes>> Except for a slightly slower response, it gives identical readings as the mercury-filled A-20 which is also manufactured by Miller & Weber.  The often ignored liquid-filled thermometer may appear to be an anachronism in this digital age but I submit that nearly all thermostats and controllers must be calibrated by one. <<Indeed, and I'd like to mention...analogous to using a quality lab-grade hydrometer to calibrate/determine the amount of "differential" in your choice of tools used to measure salinity/specific gravity>> Regards, Paul <<Thank you for sharing.  EricR>>

Digital Thermometer preference? 1/26/07 <Hello, GrahamT with you tonight.> I was on your site to see if you recommended a specific brand of digital thermometer, but was unable to find a FAQ that answered my question. <Isn't one.> I purchased a Coralife digital thermometer in early January and exchanged it for another one since the temperature was showing about 3 degrees cooler than my original mercury thermometer.   <Mercury, or alcohol? Silver, or blue/red?> When the Coralife replacement was still showing about 2 degrees cooler than the mercury thermometer, I added another mercury thermometer to the tank and both read 76 degrees (my heater is also set at 76 degrees as well).  I sent an e-mail to Coralife (esuweb.com) on 1/18 with no response from them. Not sure what you think of this product, but my opinion is pretty much made up regarding Coralife's entire product line.  I wanted to know if you recommended a specific brand of digital thermometer. <I like Coralife just fine, but I know what I like them for, and I make no bones about it. They make affordable products that live up to my expectations of them. They are inferior to many other higher-priced competitors' products, but you get what you pay for. I have personally noticed several Coralife dig. thermometers giving variously erroneous reading of temperature in the SAME SYSTEM. I'm not sure if they are always at the same margin above or below actual temp, but I don't bother with it. I have an indoor/outdoor thermometer that I bought at RadioShack that talks and yells at me when the temps go above or below a customizable preset value. I spent less than $30 dollars on this contraption. (I did coat the metal probe with plumber's GOOP and also the wire as it enters the back of the probe) If you need super-accurate, get a temp controller that tells you the temp and keeps the heater on that loop as well. For cheap thermometers, Hellolights.com has a $5 fully-submersible digital that I dint to be very accurate and convenient. HTH GrahamT P.S. BobF, now we have the first entry in the new temp-measurement FAQ ;) > <<Yet another Sub, sub, sub FAQs file to make...?Aiyeee! B>>

Re: Dig. Thermometer Preference? Follow-up 1/29/07 Thanks for the info and I agree with your assessment of the company.   <I will add that I don't hesitate to use their lights, air-pumps etc. (I think it's all the same company now anyway... Central PET Pet pet...)[like "Pigs in SPAAAAaaace"] ;) > I guess my frustration is with the company not even having the good sense to reply to one of there customer's e-mail.   <A huge no-no in my book as well. I would probably go so far as to call someone and see where it gets me.> To answer your question my original thermometer has a red line (I get your point it's not mercury as I had described).  I also had the same experience you described below, before I returned the original digital thermometer I had both in the tank and they both had different readings.   <Not very useful in our quest for duplication of environs, hmm?> I love your site, used it often over the years and I never hesitate to point others to it as a source of reference. <Thanks for the follow-up, Nick. I always like hearing back from post-ers after the fact. You should consider those little "Hellolights" digi-gauges. They seem plenty reliable to me! -GrahamT> Nick  

Floating thermometer (broken... metal ball-ballast in tank...) Mr. Fenner: A floating thermometer broke in my salt water reef aquarium and I think some of the metal beads may still be in my aquarium. Should I be especially worried? If this could lead to a disastrous consequences, what should I do to remedy this problem? Thanks, Vince <Yes to being worried... perhaps lead, maybe ferrous... at any length, bad news... if it were my tank, I'd siphon out all the possible gravel areas and CAREFULLY sort through (sort of like rice, beans, what have you, ahead of cooking for rocks, sticks...) and then rinse it before replacing (freshwater is fine here). Bob Fenner>

Thermometers & Substrate I have 2 questions for you all today 1. which is better as far as thermometers go the glass ones that go inside the tank or the sticky ones that go on the outside? <IMO, the glass ones are better, but should not be allowed to float around in the tank waiting to get broken.> 2. When I bought my substrate (crushed coral) the guy at Elmer's LFS said NOT to rinse it, but the bags say to do a light rinse. <Crushed coral needs rinsed like crazy.> Which is right? <Are you sure you have crushed coral though. Being from Pittsburgh and knowing Elmer's, I am not sure they carry crushed coral anymore. They do have various grades of sand, some larger than others.> It is not live sand. Thanks, Colleen Pittsburgh, PA <By the way, did you make Bob's pitch at Elmer's on Saturday morning? -Steven Pro>

Thermometers I have read that it is not a good idea to put anything metal in a saltwater tank.  I have a 110 gallon reef tank.  I am shopping for new thermometers and have come across stainless steel and titanium types that look interesting as I am looking for more precise accuracy.  I was wondering what your thoughts were on these items and if they would release metal ions that would harm the tank inhabitants.  Thanks for your time.   Abby Kengersky <Titanium? Wow... I would select for a good glass encased thermometer and not be too concerned with absolute accuracy. More important to be precise (get about the same reading each time), that is, to do your best to measure and maintain stability. Bob Fenner>

DOH! I Broke the Thermometer, Now What?  >Hi Bob and team.  >>Hello, Marina tonight.  >I'm afraid I've had a disaster in my 200 gallon reef aquarium. While recently double checking my chiller reading, I stupidly left a thermometer in my sump. I found it this morning, broken at the pump intake.  >>Oh my, thusly the term "disaster". It's not as disastrous as you think, though.  >The lead balls have been sucked into the pump and the mercury is gone.  >>Not mercury anymore, my friend, usually alcohol (with a dye) is used in modern thermometers.  >Everything in the tank looks OK so far (corals and fish), but I can't imagine that'll last. What should I do? I'm sure water changes, carbon and PolyFilters will help but I can't imagine I'm ever going to find the lead balls?  >>No, I don't imagine you will, either. But I wouldn't expect such a small amount to be a very big problem in the short or long run anyway. If you're very concerned about the contents, contact the manufacturer, but to the best of my knowledge the potential for mercury would be the biggest issue and as far as I know it hasn't been used for quite a few years. You're correct, water changes, carbon, and PolyFilters will help, though I don't know at all how readily lead actually dissolves in water (thinking of wrecks of Spanish galleons and all the lead shot/balls they find, all encrusted with stuff).  >Any advice you can give would be great. I hate the thought of tearing down my tank and starting again. Dave.  >>No, no, no, I really don't think you'll need to go so far. Between the water changes and the chemical filtration you should be able to deal with the small amount of dye released. For "next time", get a bit of clear plastic tubing, the kind used for undergravel filter lift tubes, along with caps. The caps can be the same clear plastic, or PVC that fits. Glue one end (I'd use Superglue-cyanoacrylate) on permanently, leave the other so you can slip it on and off. Drill some holes in the tube, and it will protect the future thermometer from such terrible mishaps. Marina 

External heat from pump Crew: <Hi Rich how are you tonight? MacL here with you> In order to give my wife back the living room, I am relocating my 55 gallon FOWLR in the basement. <Nice of you but I bet she misses the tank when it's gone.> Currently, I have four fish in a Rubbermaid container with some of their LR and a powerhead, the 55 is filled with water, sand and the rest of the LR, and my Iwaki 40RLT is running on a closed loop.  The problem is the heat. The ambient temperature is 74F, and the tank is running around 82F! <I have to say when I got this email I was very surprised. I have not heard of an Iwaki running that hot. I checked with another WetWebMedia person and they said the same thing. They also felt it was pretty much impossible with the set up that you have, to have that large a jump in temperature.> I have not even turned my lights back on yet.  My pump has been is use for about a year. I have it installed on a small homemade stand in back of the tank, about 3 feet high, to lessen the head pressure. <The thought was perhaps you had the pump inside a cabinet or other enclosed place where there wasn't proper ventilation. In that case, it might have caused a larger that usual jump in temperature but as I understand this you have your pump in an outside area. The next thought is that perhaps your you need to check the calibration of your temperature gauges. I can speak personally to this one. I usually run three or four temperature gages at a time on my tanks.> The intake is installed over the top on the right side, using two 90 degree elbows.  The output goes up about one foot, splits with a "T", and runs to both sides of the tank, and up and over with elbows again.  I have read that Iwaki's, and external pumps in general impart less  heat, so I was surprised to find this much of an increase. I do not have any previous temperature readings with only the pump running, so I am not sure if this has always been the case. <I really would check your thermometers, a rise of perhaps three or four degrees is more normal and Iwaki is pretty much known NOT to have that kind of rise.> The pump is not making any loud noises or anything. Any ideas?  I was looking forward to more stable temperatures in the basement. I am trying to hold onto that dream! <Check the calibration of your temp gauges, both inside the tank and outside of it. If when you check it you find it really is that large a jump please let us know again.> If I read wrong, and Iwaki's DO impart heat like this, can you recommend a brand that does not? <I think you have the best Rich, Please let me know how this turns out. Take Care, MacL>  Thanks, Rich

Can the sand bed be a different temp. from the rest of the tank? Hi Adam, < Hello Narayan. >  Hope all is well at your end. My reef tank has been running pretty well so far... Unfortunately I can't say the same for the newly set up refugium. What a pain to work on an under tank refugium. So, last week I tore it down and re-set it up on the floor next to the display -doesn't look as neat, but way easier to work on, plus all my aquarium maintenance stuff can go back under the display.  < If you don't mind the look of it on the floor, good idea. >  I have two thermometer strips glued on to the glass sides of the refugium, one near the bottom of the 6" DSB and the other in the middle of the water column. The water temp is 79F, but the bottom of the DSB is at 71F.  < I don't believe it. I don't believe they can be that far off. I'd get a regular thermometer. > <<I do believe it... a simple experiment/demonstration will convince you. RMF>> Should I be concerned about the refugium's DSB's temperature? The DSB in the display is 4.5" and is at the same temp as the rest of the tank.  < Just thinking of how well water transfers heat, I can't believe it has the kind variance in a tank. Here is what I would do. I'd go to Radio Shack and buy a digital thermometer. They are under $10 and fun to play with. Then I'd move that probe up and down in the water and see what it is says. > Thank You, Narayan < Blundell >

Can the sand bed be a different temp from the rest of the tank? continued But Adam, There is very little water movement in the sand bed... I can actually feel the difference with my hand against the glass. The water is noticeably warmer than the sand near the bottom.  < This just doesn't sound right. I guess it could happen, but the way glass conducts (transfers) heat I wouldn't think it would have a difference you could feel. I guess I was wrong. >  And, I'll go get the radio shack thermometer to try... The thermometer strips are on the outside of the glass and it is cold here in Rhode Island, especially near the floor, since the tank sits on top of a inch of carpeting on a concrete slab.  < I still think the digital thermometer will be a good addition. ><<As a side note, input, there are indeed some very large differences in temperature through aquarium substrates at times... for "European" aquarists (and others) the rationale for using heater cables, situated in/under the substrate... heat rises... RMF>> Thanks, Narayan

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