Are you keeping them in fresh? Are they waspish or tolerant of each other? How many do you have?
I caught them in pure fresh, acclimated them to brackish, then they broke out in anchor worms, I think the frehwater staves it off. Then I went on another trip for 10 days and came home they were all dead. But I know where to catch them now, and will in the near future get some !