Pacu

Native to the Amazon and Orinoco river basins, Colossoma macropomum is one of the largest characins on the planet. Juveniles are silver-bodied with faint dark markings and look vaguely like oversized silver dollars, which makes them dangerously easy to impulse buy. Adults are a different story entirely: deep-bodied, powerful, and capable of exceeding 88cm and 30kg in the right conditions.

Water parameters should sit in the soft to moderately hard range, with pH between 6.0 and 7.5 and temperatures around 26 to 28 degrees Celsius being the sweet spot. They produce an enormous amount of waste for their size, so filtration needs to be seriously oversized. Industrial-grade canister filters or sump systems are essentially mandatory, and water changes need to be frequent and substantial.

Despite being closely related to piranhas, pacus are predominantly herbivorous. Their teeth are famously human-like, shaped for cracking open hard seeds, nuts, and fruit that fall into Amazonian floodwaters. In captivity they'll eat leafy greens, fruits, commercial pellets, and legumes. Whole fruits like figs and grapes go over extremely well. Animal protein is not required and should be kept minimal.

Temperament is generally tolerable when young, but larger individuals can become assertive, especially around feeding time or if space is inadequate. They rarely show predatory behavior toward fish too large to swallow, but they will absolutely uproot every plant in a tank and can accidentally injure smaller tankmates through sheer physical size.

Browse real pacu setups on Shimmerscape to see how other hobbyists and public facilities have tackled housing one of the most demanding fish in the freshwater hobby.