You're redcaps arent showing any signs of disease. the folded dorsal fin is a sign of unhealthiness though. It could be stress, if you're water parameters are definitely good. Im gonna assume that when you take it into the petstore they check for EVERYTHING, so Im fairly sure that nitrates and nitrites arent a problem... umm... Im gonna say stress. They have stress eaze for fish I think (or am I thinking for people...?) did you notice signs of aggression from the other fish? Keep checking for fungus or parasites. I really hope this sort of narrowed down somethings.. mind, it could me just mindless ramble to you too

sorry...