The pacu grows large and quickly, and it needs a healthy diet to support this growth. While a close relative of the infamous piranha, the pacu is a voracious vegetarian -- for the most part. You need to replicate their wild diet to see them at their best in captivity.
In the Wild
Studies of the pacu in the wild have found that it eats a mostly vegetarian diet. This consists of fruits, nuts and seeds. However, while they mostly eat plant matter, pacu do also include some animal matter in their diets. This includes insects, crustaceans and smaller fish. Keep in mind that most fish are smaller than the pacu, which can grow larger than a meter in length.
The main food you feed an aquarium fish is called a staple. For most freshwater fish, flake foods fill this role. However, for the pacu, such foods are too insubstantial. However, many pet shops also sell pellet or stick foods that you can use as a staple for your pacu. Keep in mind that you want a pellet or stick food rich in vegetable matter. When feeding pellets or stick foods, soak them in water for about a minute before feeding them to discourage constipation.
Grocery Store Foods
In addition to an easy staple food, you should feed your pacu fresh green foods as a treat. This improves digestion and ensures the pacu gets all of the nutrition it needs. Pacu will enthusiastically devour such grocery store fruits and veggies as apples, banana, grapes, peas and carrots. Vegetable matter should be a major component of the pacu's diet, since this is more like the foods it eats in the wild.
You should also feed your pacu the occasional meaty frozen or live food. Most fish love bloodworms, krill and mysis. You buy these foods frozen from most pet shops. Break off a piece and thaw before giving it to your pacu. Most pacus will also enjoy the ocassional live treat, like live earthworms or even small feeder fish. Never use goldfish as feeders, since they are high in fat and a chemical called thaminase, which breaks down vitamin B and sickens large fish like the pacu.