I have one black piranha, and have had bad luck trying to give it a tankmate. It is only about 3.5-4 inches long, but it nipped the fins and eventually killed (maybe) my 6" tiger oscar. He has bitten the tail off of any fish I have temporarily put with him including a convict, a smaller juve albino oscar, and a 4" aggressive cobalt cichlid (just last night). He is only in a 29 gallon tank though (perfect for his size), so with more room and hiding and distractions you could quite possibly be ok for a while, but eventually they get hungry/angry.