I have a Pacu. He is really cool. He is currently in a tank with Oscars, and African Cichlids. He was in the same tank with (4) 4 inch red belly piranhas. During that time all the fish in the tank were only allowed to eat meat. The Pacu wouldn't eat meat at first and then he started to. Because he had no other choice. Ever since then the Pacu has turned into a carnivore and he has grown huge. I didn't think that Pacu's would eat meat but he does. One time I threw in some pinkies, and he rushed the pinkies and grabbed one and ate him. It took him a couple of minutes to eat the pinkie. I was shocked! I think it has to do with what environment you put the Pacu in, and how you train him.
Was at a fish store today while I waited for my bus. I knew that they have piranhas because I bought two there for some months ago (they died btw) so I checked out if they still got some. I read the sign and it said "piraya" so I looked into the nasty tank, it was filthy as hell, and behind the powerhead it sat a pacu, close to 2" I think.
Im thinking of buying him, but I dont want any diseases in my RB tank + hes much smaller so Im not shure he will get accepted.