Although some large predatory fish (snakeheads, BIG catfish, gars, pirahna etc) do occasionally eat small mammals such as aquatic rodents they cannot digest mammal fats and so have to regurgitate them to eject the fats from their systems. While this doesnt harm the fish it can cause serious water quality problems and the released stomach acids can damage the bacterial colonies. If you want to feed your fish with kitchen scraps then small strips of lean raw chicken breast and pieces of white fish can be used, but this is ONLY for large predatory fish.