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I'm interested in what you all believe to be the "right" level of salinity. The folks at 3 different LFS have told me that it is better to keep the level around 1.020 - 1.021, and this is what they do at home. They have also told me that in the stores they keep it even lower, around 1.019. I'm running mine between 1.020 - 1.021. Thanks for any input...

This is all very interesting, but from my experience, an average of 1.023 should be sufficient. Using the lower SP to rid fish of parasite may work, but I find it a bit risky and should only be done as a last resort. I live in the Caribbean where the water is 1.026 and some of my fish I've actually caught myself, but I've found that other fish that are not indigenous to here does not fair as well with this higher level of salinity, so over the years I've arrived at the level mentioned above. Most of my fish range from ages 7 years - 9 years old and recently, after getting a faulty SP tester, my fish were accidentally living in 1.018 which led to the death of my French angel from internal failures, before I realised what was happening. Prior to this, I haven't lost a fish in many years. There's a lesson there somewhere. I've now gone electronic to check my salinity. You'll learn all this in time. (Nothing beats experience…).

There are many opinions to what is the right salinity. It depends on whether it is a reef tank or not. Do you have inverts and coral? Are you over populated? What kind of fish do you have? The current thought today is to duplicate what the fish and inverts have in their natural environment. The ocean ranges from 1.025 to 1.028+. Most fish and inverts come from 1.025 to 1.026 (35ppt). Fish from the Red Sea normally live in a higher salinity.

Can you keep fish in a lower salinity? The answer is yes. I currently have an Achilles Tang, Blue Chromis, Firefish and an unknown small fish from the Florida Keys in water at 1.009 in quarantine. The Achilles had a parasite that was resistant to formalin, copper and hyposalinity above 1.012. I have had fish live at 1.009 for months with no visible adverse effects. The O2 levels can be much higher and it is less stressful due to the fish's internal fluids being close to 1.008. I have treated a yellow tang that was over 30% red with large botches and streaks along its entire body (couldn't move its tail) and the current Achilles that lost most of its mucus and pigmentation and was lying on the bottom for 2 days from a parasite that covered over most of the fish with hyposalinity and antibiotics. At normal salinity, the fish would have died.

A normal question would be why don't we keep all fish at this level? The normal response is that it is not natural. Fish have evolved and survived for 100's of thousands to millions of years in the ocean. With our level of knowledge being very low, we try to duplicate their environment to the best of our abilities, not modify it. Lower salinity of 1.019 for fish only is probably fine. Many stores and hobbyists keep a lower salinity without any long term effects. It is not acceptable for inverts, live rock and sand. These organisms can not adjust to different salinity. Fish maintain their internal body fluids at a given level. Inverts and most other crustaceans can not.

This is not to say that there are no exceptions. While camping in the Florida Keys with the Boy Scouts, my son collected some snails and hermit crabs over 2 days. These animals lived in the small pools of water that remained after the tide receded. There was also some light rains while we were camping. I measured the salinity and found that the water that these snails survived in changed from 1.012 to 1.044 (due to evaporation) in less than 6 hours. I have had shrimp and snails that died in salinity changes of 0.004. To sum it up, the current thought is too duplicate the natural environment to the best of our abilities. PS. In the 1970's, the protein skimmer was not needed (it was experimental and the testing that I read concluded that it was probably not needed) besides being unaffordable. In the early 1990's you couldn't have a reef without a wet/dry. Today you don't want a wet/dry with a reef.

Well, I agree with ME. IMHO, hypo-salinity (low salinity) is acceptable to treat sick fish as a short-term treatment, but is not a good thing for the long-term health of your fish (or inverts).   The salinity of natural seawater is approximately 35 PPT, which calculates to a SG of 1.024 at 82°F, or 1.025 at 76°F   May I suggest you read the following links:

Thanks for all the input. Sigh... Why do these guys at the stores always give the wrong info? After this discussion, I agree that for a retailer bringing in new fish all the time, low salinity may help with bacterial outbreaks, etc, but doesn't look to be good for a long-term environment with long term occupants. You would think they would have the right advice for something that is as basic a requirement as this. Oh well, I found the advice for reptiles and amphibians to be wrong most of the time as well. I'll be raising the salinity slowly over the next few water changes until I get it up where it belongs. Don't want to over-do it too soon.

Raising the salinity safely is easy: Just add half strength SALT water to your tank when doing top-ups instead of doing it with fresh water. The salinity will slowly increase because the salt remains in the tank when the water evaporates.