This is exactly why I refuse to buy fish kept in low salinity. Store owners say it prevents disease. IME it suppresses disease then when you get the fish into normal salinity the disease rears its ugly head. I had the exact same thing happen a few years ago. Bought a fish in low salinity and after a few days velvet started killing fish in my tank. I managed to save a few but lost many. I no longer purchase fish unless they are in a tank with coral or inverts or both.im starting to come to the conclusion it might be velvet. i just didnt think it would happen that fast though.