This is a pacu, one of the classic fish of the Amazon river basin. Sometimes it's called a "giant piranha" but in the Amazon it is a vegetarian, preferring fruits and nuts to flesh. (Supposedly, a population introduced to Papua New Guinea where the Pacu may not have its normal food, eats meat and attacks people.) It belongs to the Subfamily Serrasalminae and probably the Genus Mylossoma or Myleus. It was also our dinner.