Our local Petco has a humongous pacu that must weigh at least ten pounds in an outdoor style freestanding glass pond. I understand that the pacu is a copycat to the pirhana, which I have seen at Seaworld in San Antonio, and the adult pirhana's were about four pounds. Is the pacu I am talking about a genetic freak or do they actually get this large? It seems that the disguise would be less effective if they are not the same size as the fish they imitate.

Scientific name …colossoma brachypomus (silver) or colossoma macropomum (red-bellied)

The pacu originate from the upper Amazon, and they can grow to be between 18 and 30 inches in length,

They are related to a piranha, but not a piranha per se. Those who own this breed of fish say that they are a friendly breed and intelligent. The only time they act like a piranha is when they are small, but this is merely a protective behavior until they reach a certain length then this behavior ends.

It also has been noted that while pacu are mostly a vegetarian, they will if given the opportunity, eat smaller fish in the tank. Some owners say that they get along well with other community fish, while others have reported missing fish.

Interesting, this one is at least 20 inches long and six inches thick I would estimate. It is a massive fish larger than any Oscar I have seen. In fact I thought at first glance that it was an Oscar, but on second glance it was clearly a pacu. It is in a tank with koi and a few Jack Dempseys. The JD's do not mess with him though!

Thanks piggins, for the good answer!