test your water parameters. salt is much more involved chemically than fresh. whats your sg at? is your ph at 8 or higher? do you have ammonia in the tank? what about the temp of the tank? you should really test everything to make sure that its not something that you are doing/not doing in the tank. usually if you do that you find out that one of the levels is a tad off you can fix it before more damage is done to the fish