I have a really big Pacu (looks like a piranha) that has outgrown my tank. He is about 14-15 inches long. He is currently in my tank with two Oscars and gets along fine with them. Now, I feel sorry for him because he doesn't have enough room to barely turn around. If anyone is interested, let me know............
http://www.petfish.net/articles/pix/...molly-pacu.jpg