i feed my fw fish some sort of seafood daily and they love it. its totally fine except tuna, if i remember correctly tuna has extremely high levels of iron or zinc. too much could kill.
My grocery store always has some Special of the day fish....its usually some Tilapia fillets, my fish love them. I like to take the tilapia and run it threw a food processor with some spinach and some plain gelatin and then make my own frozen fish food cubes...its very good for them I dont like to feed oyster since its kind of oily it makes the water a bit dirty for awhile. Long as the fish hasnt been previously cooked, salted, breaded or anything to that affect then IMO its perfectly fine to give them.