Order Characiformes
Family Characidae (American Tetras)
Subfamily Serrasalminae (Piranhas and Pacus)
Representatives of the subfamily Serrasalminae (literally translated from Latin as “toothed salmon”) inhabit freshwater bodies across the tropical regions of South America. The word “piranha” originates from the Tupi language, derived from “pira” (fish) and “ranha” (saw). Knowing a little about the habits of these fish, one cannot help but agree that their name is well-deserved. They are found in all freshwater habitats from the eastern foothills of the Andes to Colombia, Venezuela, and Guyana in the west, across the entire Amazon basin, in Bolivia, Peru, Paraguay, Uruguay, and northeastern Argentina. Carnivorous fish capable of stripping their prey clean in mere seconds account for about 40% of the entire family. The remainder prefer a herbivorous or omnivorous plant-based diet.
Within the subfamily Serrasalminae, three main groups can be distinguished: the first group of herbivorous fish “Pacu” (Colossoma, Mylossoma, Piaractus), the second group “Myleus” (Myleus, Mylesinus, Tometes, Ossubtus), and the third group of carnivorous fish “Piranha” (Serrasalmus, Pygocentrus, Pygopristis, Pristobrycon, Catoprion, Metynnis).
These fish possess a deep, laterally compressed body, long anal and dorsal fins, a distinct adipose fin, short pelvic fins, and a slightly notched caudal fin. Thanks to their large and powerful tail and very fine scales covering their streamlined bodies, they are exceptionally fast and agile swimmers. Body coloration varies from silvery-blue with dark speckles to dark gray with sparkling flakes. Juvenile piranhas feature a dark band along the tip of the tail and the caudal peduncle. Males are usually bulkier and more brightly (often darker) colored. Most species are predatory.
Piranhas are armed with razor-sharp, blade-like interlocking teeth. The teeth are arranged so that when the jaws close, virtually no gap remains between them. A piranha’s jaws are so strong that a single bite from an adult can cleanly sever a stick as thick as a human finger. However, they only turn to frenzied predation in stressful situations: starvation, the drying out of temporary pools (which shrinks living space and deteriorates water quality), or when placed in unusual conditions (cramped aquariums, fright, transport).
Life in a piranha shoal is filled with constant fear and mutual distrust, even when everything appears calm. Every individual is capable of severely injuring or killing another. To survive, a fish must constantly monitor the whereabouts and actions of its tankmates and anticipate their next move. This behavior partly explains why many aquarium-kept piranhas remain surprisingly skittish and nervous, despite their fearsome reputation.
The majority of carnivorous piranhas are highly aggressive, intolerant of conspecifics and other fish, and therefore not recommended for novice aquarists. Most hobbyists begin with Pygocentrus nattereri, Serrasalmus rhombeus, or Metynnis maculatus, as these are the most widely available and easiest to keep. When purchasing piranhas from an aquarium shop, make sure they are active and free from severe damage such as a missing eye, deep open wounds or ulcers, or erratic swimming behavior. Missing or torn fin edges are generally not a serious issue (nipping happens frequently, and fins regenerate quickly).
They require a large aquarium with plenty of plants (live or artificial) to serve as shelter. In addition, driftwood branches and large stones provide extra cover. They are best maintained in species-only setups. Juvenile piranhas can temporarily be housed in relatively modest tanks (80–120 liters, depending on the species and number of fish), but young fish grow very rapidly (especially Pygocentrus). Once they reach seven to nine months of age and 10–15 cm in length, they must be transferred to a significantly larger tank.
Keeping piranhas is not particularly difficult, but their excessive aggression prevents them from being kept with community fish. Even amongst themselves, they do not always get along: particularly in stressful situations, rivals can inflict severe injuries on one another. Despite their formidable reputation, piranhas are skittish; in panic, sometimes without obvious cause, they may dart blindly and crash into rocks or other hard decorations.
Piranhas are best kept in a shoal of 5–8 fish of identical size and age. With regular feeding, they will tolerate each other relatively well while strictly maintaining a hierarchy. Armored catfishes can sometimes coexist with them. Once the piranhas have fully acclimated to the aquarium, introducing select tankmates can be considered, though success depends largely on tank volume, fish numbers, and aquarium layout.
Piranhas are opportunistic predators, and it is not unusual for them to suddenly kill robust tankmates that have coexisted peacefully with them for months. Outbreaks of aggression are frequently triggered by stress or overcrowding. In addition, higher water temperatures increase aggression levels, so dropping the temperature by a couple of degrees can often help reduce hostility. Finally, well-fed piranhas tend to be far less aggressive than hungry individuals.
While members of the serrasalmin line are predominantly carnivorous, most myleins are strictly herbivorous. Aquarium decoration should utilize plastic plants (for herbivorous species) or robust, firmly rooted plants (for predators), as spooked fish dash violently around the tank, easily uprooting fragile foliage. Fine sand or small gravel (2–5 mm) works best as a substrate. Piranhas dislike excessively bright lighting, as it makes them feel exposed and vulnerable.
They can adapt to varied water parameters and even survive in murky water in nature, but optimal conditions should be provided in captivity: temperatures between 24–28°C and a pH between 6.0 and 7.5. The water must be clean, and weekly 30–40% water changes are vital to maintain vigor and healthy development. A powerful and efficient external canister filter is an absolute necessity, accompanied by a powerhead to provide strong water movement and oxygenation.
Juvenile piranhas can be fed small portions two or three times a day, whereas adults only need feeding once every 2–4 days (healthy adults can easily fast for a week). Extreme caution must always be exercised during tank maintenance. They reach sexual maturity at 1.5–3 years of age and can live up to 10–12 years.
Genus Acnodon.
— Sheep Pacu — Acnodon normani [Gosline, 1951]. South America: Brazil, Amazon, Xingu, and Tocantins river basins. 13 cm. Coloration is greenish with a silvery sheen and faint vertical bars on the flanks. The first rays of the dorsal and caudal fins are elongated. The caudal fin is deeply forked. They require strong current and pristine water. They are swift swimmers that may injure themselves on hardscape or leap out of the aquarium if startled. pH 5.8–7.2; dH up to 18°; T 22–28°C.
Genus Catoprion.
— Wimple Piranha — Catoprion mento [Cuvier, 1819]. Synonyms: Serrasalmus mento, Mylesinus macropterus. Colombia, Venezuela, Guyana, Brazil, Bolivia: Amazon, Rio Negro, Orinoco, Essequibo, Paraguay, and Madeira basins. 10–15 cm. Features an upturned, elongated lower jaw. Greenish-gray body with iridescent specks, a large red spot on the gill cover, and a reddish belly.
The outer rays of the dorsal and anal fins are prolonged into filaments; the base of the tail is black. Fins are grayish, with an orange anal fin. The caudal fin is black at its base. Mature specimens develop remarkably extended anterior dorsal rays (hence the name “wimple”), which can break during capture or transport and sadly do not regrow. Males have a convex anal fin profile.
This species is unsuited for community aquariums. It is a specialized predator: small fish will be eaten, while larger fish will quickly have their fins and scales stripped away. With its blunt upper jaw and specialized scrapers, the wimple piranha is lepidophagous (feeding primarily on the scales of unsuspecting neighbors). Only juveniles can be kept together; adults must be housed singly. As females mature, they will persistently attack males.
They swim very swiftly. They thrive in a spacious aquarium decorated with floating plants, individual driftwood roots, and clusters of vegetation. Water flow should be gentle. They are sensitive to poor water quality and sudden fluctuations in water chemistry. Perform regular, modest water changes rather than large replacements. In captivity, they feed on chopped shrimp, mussels, and other meaty foods, supplemented with vegetable matter. dH up to 15°; pH 5.8–7.2; T 23–28°C.
Genus Colossoma.
Maintaining these fish requires immense aquariums equipped with large hides. They feed primarily on plant matter. They eagerly consume almost any aquarium plant, even testing artificial ones; Java moss is perhaps the only exception. They prefer soft to moderately hard water with a near-neutral pH. Optimum temperature is 23–28°C. Staple diet includes cabbage, lettuce, spinach, nettle, peas, and similar vegetable matter. If raised exclusively on animal food, their offspring will be very weak or they may fail to breed altogether. Spawning is stimulated by fresh, soft, slightly acidic water.
— Black Pacu, Tambaqui — Colossoma macropomum [Cuvier, 1816]. Synonyms: Colossoma nigripinne, oculus, tambaqui; Myletes macropomus, nigripinnis, oculus; Piaractus macropomus; Melloina tambaqui. (Also known as Black Finned Pacu, Cachama, Gamitana, Giant Pacu, Red Bellied Pacu). Colombia, Venezuela, Peru, Bolivia, Brazil: Orinoco, Amazon, Guanare, and Meta river basins. 90 cm. This is the second-largest fish species in the Amazon basin. They possess immense jaw strength. They migrate along major river channels but spend most of their time feeding in flooded forests and shallows.
Juveniles display small dark spots across a silvery-steel background; some varieties feature nearly black fins, while others show bright pink fins and chests, making them very attractive in display aquariums. Unfortunately, as they mature, the vivid coloration fades into a chocolate-grayish hue with gray flank patches. Scales are fine and dark gray. The abdomen and anal fin are almost black in adults, while orange in juveniles. Males feature an expanded dorsal fin, a serrated anal fin, and brighter coloring than females.
Peaceful giants, they spend most of their time hovering midwater or near the surface in groups. Housing them requires massive aquariums with heavy glass exceeding 800 liters, combined with heavy filtration and aeration. Tank volume and filtration capacity are far more critical than decorative elements. Always ensure plenty of open swimming room. When startled, these skittish giants can overturn heavy decor and have even been known to crack front aquarium panels.
Herbivorous despite their striking resemblance to piranhas. They will rapidly strip any aquarium bare of greenery and grow astonishingly fast—easily reaching 2–3 kg in a home aquarium without requiring a public-aquarium-sized tank to do so. They are nervous and dislike cramped quarters. A massive biological filter is vital to process the substantial bio-load produced by a fish of this stature.
In the wild, they feed on fallen fruits, nuts, and seeds; in captivity, they accept peas, cichlid pellets, and various fresh greens. Particular attention must be given to vegetable supplements, which form the bulk of their natural diet. Lifespan reaches up to 20 years. T 25–27°C; dH 5–8°; pH 6.4–6.8.
Identification: Black Pacu — Colossoma macropomum.
Mylossoma aureum, duriventre, Myloplus rubripinnis, Myleus schomburgkii, ternetzi.
Mimagoniates barberi, inequalis, lateralis, microlepis. 
