Piaractus brachypomus--red belly pacu. As they mature, red bellies usually lose what little red they might have. Males generally have more red coloring than females. They also have longer dorsal fins and their anal fins are concave as opposed to convex. I would say you have a male red belly pacu....a handsome one.
Keep in mind that pacu are long lived fish and can live 30+ years, but can die in 2 years with poor water parameters. They are more sensitive to nitrogen products, so keep up with the water changes. While red belly pacu do not get as long as black pacu, they can still get to 3 feet in length. Their shape is more rhomboidal than the elliptical black pacu, so red belly pacu can get quite "tall" and can have tremendous girth. I would say a minimum tank height would be 3 feet, but 4 feet would be better.
Our pacu were 15" when they came into our lives. They had no red, so I assumed they were black pacu. WRONG.
Mature black pacu can actually have more red coloring than adult red belly pacu. This pic was taken off the internet. Note the different shape (elliptical) of the body and the elongated head, and the larger operculum, and the green/red not ever seen on red belly pacu.
John PTC's black pacu:
My rotund red belly pacu