Pacu are members of the Serrasalminae sub-family of Characins. The Characins is the group of fish, primarily from South America and Africa, that is best known as the Tetras. The Serrasalminae sub-family of Tetras contains the Pacu and their close cousin, the Piranha. These fish are from South America and have many similarities in both appearance and behavior.
Pacu are generally very large fish, smaller species reaching around six inches (about fifteen centimeters) and larger species exceeding two feet (sixty centimeters), with record-setting fish over three feet (ninety-two centimeters) in length. These fish generally live for four to twenty years, depending on the species and the quality of care provided for them (larger varieties tend to live longer).