[leebca]

The answer to that one hasn't been totally figured out. There is a lot of speculation, but the fact is that marine fishes have lived in hyposalinity for years. Is this good for them? It isn't conclusively known. Most professional aquarists are 50/50 but veterinarians and the scientific community say that the hyposaline state is not good for the internal chemistries (physiology) of the fish. Still, fishes have been kept alive in this low salinity for years.

Some fishes are more tolerant of it than others. About 1 in a 100 don't seem to be able to handle it even for a short time. It's not a species problem, but an individual fish intolerance.