How long have your had them ? They are mainly carnivores, but will feed on most flake food and sinking wafers. Though ideally it should be mostly protein based and not vegetable based. Things such as the Tetra Pro Crisps, Hikari Carnivore Pellets seem to go down quite well. And if you have any doubt about their feeding, give them some live (or frozen thawed) bloodworm. If they don't go wild when you put this in the tank, you will have some cause for concern.