Yes it is a Red Belly Pacu, close relative to the Red Belly Paranha. You can tell the difference in the jaw shape. Piranha's will have a small row of teeth barely visible in their mouth, Pacu's wont.
Pacu's are herbavorious fish, they love plant matter. I.E. Give him a nice slice of Zuchini 3-4 times a week, remove the slice after 24 hours. You'll be amazed of how much they love Zuchini.
WARNING, THEY GET MASSIVE IN SIZE, UP TO 2-3 FEET IN LENGTH.