[6] It is similar in shape to the piranha and juveniles are sometimes confused with the carnivorous fish; the tambaqui is tall and laterally compressed with large eyes and a slightly arched back.

Unlike more predatory species, the teeth of the tambaqui are molar-like, an adaption for crushing plant seeds and nuts.

The tambaqui resembles the Red-bellied pacu (Piaractus brachypomus), but the latter species has a more rounded head profile (less elongated and pointed)[8] and a smaller adipose fin that lacks rays, as well as differences in teeth and operculum.

[6] During the non-breeding season, adults stay in flooded forests of white (várzea), clear and blackwater (igapó) rivers.

[2][6] They stay there for four to seven months during the flood season, but as the water level drops they move into the main river channels or to a lesser extent floodplain lakes.

[6] The schools then break up as the adults return to the flooded forest of white, clear and blackwater rivers, and the annual pattern is repeated.

[4] Juveniles stay near macrophytes in floodplains and flooded forests year-round, only switching to the adult migration pattern when reaching sexual maturity.

[16] Another study of the stomach content of 138 specimens during the high-water season found that 44% of the weight was fruits and seeds, 30% was zooplankton and 22% was wild rice.

[6] In addition to seeds, fruits, wild rice and zooplankton, smaller levels of insects, snails, shrimps, small fish, filamentous algae and decaying plants are consumed.

[4] Based on a review by IBAMA, it was the 11th most caught fish by weight in the Brazilian Amazon in 1998 (just ahead of the closely related pirapitinga, Piaractus brachypomus).

[24] In fish farms this species is sometimes hybridized with Piaractus to produce offspring that accept a wider temperature range (colder water) than pure tambaqui.

[25] There is also an introduced population in Puerto Rico and singles (likely deliberate releases by aquarists) have been caught in a wide range of U.S. states,[26] but only those in the warmest regions can survive.

[27] Juveniles 5–7.5 cm (2–3 in) long, sometimes labelled as "vegetarian piranha", are frequently seen in the aquarium trade, but they rapidly grow to a large size and require an enormous tank.