Piaractus brachypomum is most often called Pirapitinga and Cachama world wide, though in North America, they are most widely known as the Red Belly Pacu (or sometimes Paku). When young they are somewhat similar in looks to Pygocentrus nattereri or the Red Belly Piranha and because of this, frequently called a mimic. Perhaps in part because of the popularity the Piranha, Red Belly Pacus have also long been very popular for the home aquarium. Some times they are seen as a “safe” substitute for their similar looking carnivore cousin as Piranha are illegal to own in many states. There are pluses and minuses to keeping both of these interesting fish, but Piranha husbandry is another article.
Piranha and Pacu are easily differentiated by looking at the jaw and teeth. Piranha have a single row of very sharp, “arrowhead” looking teeth and an underbite. The Pacu have teeth that have gained them some internet fame for looking a lot like human teeth, and there is a good reason for this. They primarily eat fruits, nuts and vegetation, and as such their teeth are set up to do this job well (just like ours). They have a powerful crushing bite to deal with the hard nuts they frequently consume, as a result they occasionally bite unwary fishermen, which is allegedly very painful and in the case of large fish, can cause serious trauma to small extremities. Long story short, (and we feel this is a good rule for most fish) Don't put your finger in their mouth.
Like most “Pacu,” the Red Belly Version can get very large, the biggest on record being almost 3 feet long, 30 years old and 55 lbs. In warm conditions with adequate food, this growth rate is very fast, and is certainly one of the only reasons an average home aquarist shouldn't keep one. They can be more than a foot long in less than a year, meaning most aquariums will not offer them adequate space. In cooler water, say under 70 degrees, this growth rate is inhibited dramatically and most specimens will struggle to reach more than 5 or 6 inches long in the same time frame; they will still attain very large sizes, but at a slower rate.
So they bite and get huge, why would someone keep an animal like this? Pacu are remarkably good aquarium inhabitants, they are very hardy and can live in a wide variety of conditions. They are quite “personable,” disease resistant and adapt well to common aquarium foods, especially large pellets which they will take vigorously as they mimic a natural food source in floating nuts and fruits. Real fruits/nuts/vegetable matter should also be a part of their captive diet; keeping the protein levels low in their diet helps slow their prodigious growth rate, and results in a healthier animal. They can be kept in groups, especially in massive aquariums and ponds in warmer climates, though they do tend to become more solitary as adults. Their penchant for eating stray vegetation combined with their engaging behavior and massive adult sizes makes them a popular pond fish. They are also a prized and important food fish in their native range. They have a firm white meat that is almost pork like in consistency and is frequently barbecued as a result. For you Piscivores, if you ever see them on a menu, be sure to try them! Most of the pacu available are aquacultured though occasional wild caught specimens are seen in the US.
So do you want a Pacu? If they answer is yes, think about your long terms plans for this fish. Do you have a large pond in an environment they can live in year round? Are you the kind of aquarist who also loves eating fish? Do you have plans to put a pond or tank that is hundreds if not thousands of gallons in the house somewhere? The only real catch to keeping this wonderful fish is what happens later. Like any non-native fish, they should never be released to wild waterways where they can become a serious problem because of their penchant for eating vegetation and thus changing habitats for the native fish. Consumption of overgrown fish is one of many ways suggested for safe disposal of these animals. Just be sure they come from environments that haven't been medicated with toxic substances. If you've ever eaten farmed fish, the only effective difference is that you know exactly what this one got fed! Donation to local aquariums and or other aquarists with larger aquariums can also be an option for rehousing overgrown pets. With a good plan for the future, there is almost no better aquarium fish available!