I have the same thing happen with my cardinals. Although the only
other fish in my tank are the SAEs (4). And they dont have any
problems. I just have to make sure the surface skimmer is on cause I
dont have an air pump. Surface skimmer seems to help. Before the
cardinals were dropping like flies. Lost over a dozen of them. Then
again it could be other reasons since my tank is a new set up.

Cheers from Buffalo.

*anyone want some snow?