Vast changes in SG.  9/20/07
Dear Bob,
I have a question on Specific Gravity. I have been reading lots of books, browsing the net. In some book I have read that slight changes in the SG may stress the fish and the fish may die in short run. Now I cant understand the following.
1) SG in the sea 1.025
<Most of the seas, yes... The Red Sea a bit higher...>
2) SG during Cu treatment suggested 1.017. ?
<Mmm, suggested by some>
3) SG during for Hyposalinity 1.009. ?
<Again... but not by me in most cases>
4) SG during fresh water dips. Zero.?
<1.0000...>
5) SG in FO tanks 1.020 suggested in some books: Good to keep disease away ?.....
and many more SGs....????
<Yes... many opinions my friend>
Well, if the fishes are going through such a lot of changes in the salinity during , Does it mean they are capable of the sudden changes in the SG. Like pH change should not be more than 0.2. What is the maximum change at on go the fish can handle?
Regards,
Inderjeet Singh
<Some species considerably a bunch more than others... Think about the coastal fishes that contend with river and other run-off... I have seen marines swimming freely into 100% freshwater on their own... IF they're in "good shape", most marine fishes can handle sudden and large changes (temporarily) in spg... Bob Fenner>

Permanent Hyposalinity For A Marine Tank? 7/29/04
I was wondering if it would be okay to house marine fish without any inverts in 1.008 to 1.012 salinity for good?  I've heard that low salinity is very easy on these fish and reduces stress significantly and kills off marine diseases used to 1.026 salinity (normal sea water).
<Well, there are a lot of opinions out there on this subject. Many wholesalers and transhippers may keep fishes in lower specific gravities in order to help lower the possibility of parasitic diseases. However, in the long term, such low specific gravities may not be safe. I've read of studies that have demonstrated that fishes have suffered damage to their internal organs as a result of long-term exposure to an unnaturally low specific gravity. If it were me, I'd stick to 1.022-1.025 (I like 1.025) and be done with it! Fishes have evolved to live in these specific gravities over the eons, and it works just fine. Why change things now! Regards, Scott F>

SG
HI BOB-
Quick one on SG. What is the SG of seawater on the reef (excluding Red Sea) someone says it has risen to 1026 is this accurate? After purchasing the deep six I realized that my SG was 1016 I have raised it to 1020 over 3 days and would like to maintain it at around 1022-1023 how does that sound.
Thanks
Tom
>>
Hmm, no, still closer to 1.025 (temp. adjusted) most everywhere around the world.
For reef systems, closer to seawater is better, though spg's that are lower are tolerated by most types of livestock... and there are some upsides to lower salinity... greater gas solubility, cheaper water changes, lowered disease pathogenicity... 1.022-1.023 will very likely be fine.
Bob Fenner

Issues For Discussion
Dear Robert,
<Howdy>
Hello there, it's me again!!! I have a few questions to ask you, so please bear with me.
<Okay>
Michael and I are at odds as to what the comfortable salinity should be regarding my reef tank. He thinks that I should bring it down all the way to 1.017-1.019. The SCMAS group convinced me to bring the salinity up to 1.024 and up. Will you kindly help settle this question?!? Do I have it up too high and SCMAS is wrong or is Michael's assessment too low?!? Whatever you say, I shall adjust accordingly. Every time he comes to clean my fish tanks, he drastically brings it down by adding too much fresh water and brings the salinity down somewhere between 1.019-1.021 which kills a fish or invertebrate (or so I think). What to do?!?
<By and large almost all captive marine systems should be kept near seawater spg... 1.025 or so... There are many valid reasons for keeping spg artificially low for periods of time... to save money on salt mix, reduce parasite pathogenicity, allow for greater oxygen solubility... But permanently leaving spg low has proven to be trouble... especially for non-fish livestock as you mention. I would elevate, leave yours in the 1.024, 1.025 range. Bob Fenner>
Sincerely yours, 
Aleida Ann Graichen

Low Salinity Stress?
Hello Mr. Fenner,
<Anthony Calfo here...fellow Pittsburgher, and happy Steeler fan today!!!>
I am new to the hobby and have about 6 months of experience in saltwater and my 55 gallon tank is also approximately 6 months old.
I have read the WetWebMedia boards in search of something related to this, but I could not find anything that I thought could help me.
I had a problem a month and a half ago with a Yellow Tang who developed, or was infected with parasites when I got him. I gave him a freshwater dip and quarantined him for approximately 3 weeks and he seemed to be doing good in the qt tank. One morning I woke up and turned on the light and he was dead. Anyhow, the store I bought him from had me do all the tests on my main tank where he originally was, and all the readings were ok. They said to drop the salinity 2 pts per day until I reached 1.10 s.g.. 
<Severe. Only acceptable in systems without invertebrates/live rock and dubiously extreme. Just because some fish are strong enough to survive it doesn't make it right. This methodology has sprouted from an anecdotal article which included the support of a the old Pittsburgh AquaZoo curator who does consult a local store. Very controversial.>
They also said to hold it there for a month to make sure that if there were parasites in the tank, they would not survive.
< conditionally, but not guaranteed>
Anyways, I did that and the fish looked great, but the live rock looked like it was dying and all of the worms were gone.
<not a surprise...obviously and visibly damaging to everything but the fish>
At the end of the 1 month period, I started on Wednesday slowly increasing the salinity by 2 pts per day. Everything looked ok except now my Coral Beauty seems to be refusing to eat, but my False Percula Clown is still chowing like a pig. The Coral Beauty will not eat flake, freeze dried brine, Pygmy angel frozen food, of Sea Veggies. When the clown starts to feed he swims around with him but will not eat anything. I do not know if my rock is dead or dying and it is causing ammonia to rise and that is affecting the Coral Beauty or what. 
<surely damaged your biological filter...to what degree remains to be seen>
I am also confused on why my levels are what they are. Could you please try to help me out and tell what I should do.
<at this point patience, resumption of normal routine and water changes>
Some of the live rock has turned white during the drop in salinity. I thought that the algae dying on the rock could also have led to the Coral Beauty to stop feeding, because he grazed on it a good bit.
<a small stress... the salinity was principal and more severe>
I don't know if I should pull the rock, or it will come back on its own as the local fish store says. 
I would sincerely appreciate your professional opinion on this matter.
Thank You, Jim. Pittsburgh, Pa
<it will come back on its own. After the water chemistry is assuredly stable, add some fresh live rock to inoculate the damaged rock and all will be OK in time. Best Regards, Anthony Calfo>

Blue Damsels salt levels
I currently have a 29 gal. Brackish tank, I have been told that blue damsels can live with a salt level of 1.017 my tank is currently 1.018 with 2 archers an Orange Chromide and some bumble bee gobies. I wanted to add a few damsels but am kind of afraid to add fish that I thought needed 1.020 at least. Can they tolerate brackish water? Thanks
<They can tolerate the lower salinity but are much better off being kept at true marine levels. There are a few species of Damsels that are from fresh or brackish areas, you might want to check into these. Take a look at http://home.rochester.rr.com/akom/FAQ7.htm for a list of the freshwater ones and do some research for the brackish varieties. Ronni>

Salinity and health - 12/16/03
Hi,  As I've been reading on in various books, articles, ETC. I've read that the ocean has a salinity of close to 35 ppm (sp. gr. 1.026) however the LFS (and any hydrometers I've seen) suggest that 29 ppm (1.021) is perfectly fine. <Some even state 1.018-1.019. Interferes with pathogenic abilities to thrive and survive. I don't recommend this though except in quarantine. I personally keep salinity at 1.025 in my main tanks> This got me really confused, as I would love to see these animals thriving in the best possible conditions for them, should I begin raising my salinity or is 29ppm good? <How are the animals doing?? If it isn't broken then it doesn't need fixing??>  I have a fairly lightly stocked 125 gallon A small school of Chromis, Firefish goby, cinnamon clown, yellow tang, six-line wrasse, sebae anemone, colt coral a few cleaner shrimp, and an emerald crab. Any advice would be most appreciated. <I would work towards 1.025 if there are problems or if it will ease your conscience but otherwise use your inhabitants as your barometer for change. If it ain't broke then don't fix it. ~Paul>                                                                    
-JIM