We had Piranhas in our tank here at the dorm. They were cool to watch everyone once and a while. If they weren't eating though, they just sat around. They went belly up. We then bought Tiger Oscars. They grow much faster, and are much more active. Plus, when you want the carnage, they will eat feeder fish. We put two male betas in the tank just to see what would happen. The Oscars ended up eating the Betas before there was any fight