I had two black bullheads get into a real gnarly fight. A little while ago. I believe the larger one was eating all of the food before the smaller one had a chance to get at it, causing the smaller one to get territorial. The smaller one then attacked the larger one, completely covering the larger on with wounds. The larger one did bite a huge chunk out of the smaller one's dorsal fin, though. Catfish, and other scaleless fish sure are easily beaten up, however they are also tough as nails!