Serrasalmus rhombeus (Redeye Piranha, and see below), is a fish of the piranha family Characidae found in South America in the Amazon and Orinoco river basins, the north and eastern Guiana Shield rivers, and northeastern Brazilian coastal rivers. Its length is up to 41.5 cm.
These piranhas live in very diverse habitats ranging from soft blackwater, to hard whitewater areas in the Amazon, and as such, are very tolerant of differing water chemistry.
This fish was long known and traded as Serrasalmus niger. It varies widely across its range and whether it really is a single species is still unknown, though in some cases, the fish in question are certainly mere morphs. Peruvian S. rhombeus are called jet black highbacks or Peruvian Black Piranhas. Brazilian Black Piranhas are actually greyish in color and some have diamond-shaped scales. Venezuelan S. rhombeus have the brightest red eyes and grey coloration.
Other names include Rhombeus Piranha, for its striking shape. Particularly light-colored populations are the White Piranhas of the aquarium trade, formerly believed to be the "true" S. rhombeus. A common local name is caribe ojo rojo; the species is also known as caribe amarillo, not to be confused with the other "yellow" piranhas S. gibbus and S. spilople.
In the aquarium 
It is generally recommended to keep no more than one in captivity, as redeye piranhas generally do not tolerate tankmates, particularly when they are adults. In the wild, it is believed that they generally form loose shoals (or schools, but seem to join and leave different shoals at will). Their primary source of food at a juvenile size is the fins and scales of other fish. In captivity it is possible for them to reach a size of up to 35 cm (14 in). It is rare to see them at this size, however, as their growth rate is generally very slow. Specimens that are in the upper size ranges can command very steep prices (such as $500 for a 30 cm (12 in) fish). In the aquarium they should be fed a varied diet. Many people feed beefheart, and recipes for mixtures abound on the internet. Feeding live food is controversial.
Like all piranha, they have razor sharp teeth and powerful jaws and caution should be used when performing tank maintenance, particularly when they are of a mature age. They are much bolder and extremely aggressive at an older age. Juveniles are, on average, fairly timid. Individual animal temperament does, of course, vary.
As with any aggressive species, a close attention to water parameters (i.e. ammonia, nitrates) is important, as they are very messy eaters.
- Froese, Rainer and Pauly, Daniel, eds. (2006). "Serrasalmus rhombeus" in FishBase. June 2006 version.