There is a possibility it was the Cupramine, but it may also have been the water quality. I didn't see anywhere about the water quality. I just have never seen any bad reactions to Cupramine in any fish, even copper sensitive fish. I treat every fish I get with Cupramine and still have yet to see a bad reaction. I have seen some fish be a little reclusive and slow down on eating after Prazi and it is probably what JHemdal describes as the flukes leaving holes in the fish's skin after being killed.