The only fish I wouldn't feed to my fish are carp/koi/gold fish and bullhead catfish. I have no idea why but every time I fed my tropical cats bullhead filets they either wouldn't eat it or threw it back up a few hours later. I've fed them channel catfish filets before with no issues but they really don't like bullhead filets. My flathead on the other hand will attempt to eat a bullhead twice as big as he is...go figure. The main diet for my fish is whole herring/alewives, chain pickerel filets and large (5-6 inch) fathead minnows. I try to lean towards feeding whole fish when possible.