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  • Species: Serrasalmus marginatus

  • Species: Pygocentrus nattereri

  • Species: Pygopristis denticulata

  • Species: Serrasalmus gibbus

  • Species: Serrasalmus humeralis

Piranhas : Not just killing machines

Piranhas have a reputation for being fierce and dangerous animals that will overwhelm anything they encounter through sheer force of numbers. Though each piranha might be small, they’re well armed with sharp teeth and strong jaws, and as a group they will quickly strip the meat from the bones of their unfortunate prey, reducing it in minutes to nothing more than a skeleton.

That’s the myth, anyway. In reality they are rather shy, skittish animals that spend most of the time lurking among plants and in the shadows. They aren’t especially active nor highly intelligent, and compared with cichlids or pufferfish don’t really make very engaging pets. But they certainly are interesting animals, and often rather nicely coloured as well, so for the reasonably experienced aquarist they have much to recommend them.


Piranhas are characins, and as such they are close relatives of things like tetras and pencilfish. More specifically, they are members of the subfamily Serrasalminae, a group that can be roughly divided into three basic groups as far as aquarists are concerned: the giant vegetarian pacus (such as Colossoma spp.); the smaller omnivorous silver dollars (such as Metynnis spp.); and the more predatory piranhas (such as Pygocentrus spp.).

All these fishes have the same rather rounded and laterally compressed body shape. The key differences are in how the jaws are arranged and what sort of teeth they contain. The vegetarian pacus for example have flattened teeth a bit like molars ideal for crushing nuts and seeds. Piranhas, on the other hand, have sharp teeth used for slicing flesh, and the lower jaw typically extends further than the upper jaw.

While the taxonomy of the Serrasalminae is currently in a state of flux, ichthyologists currently treat five genera as being true piranhas: Catoprion, Pristobrycon, Pygocentrus, Pygopristis and Serrasalmus. Of these, Catoprion, is distinctive for feeding extensively on the fins and scales of other fish.

Ecology in the wild

Piranhas are found in a variety of habitats including streams, rivers and lakes. Piranhas occupy a mid-level position on the food chain. While they are important predators of small fish and invertebrates, they are in turn eaten by many other animals including otters, river dolphins, large predatory fish, and people.

Contrary to the myth, piranhas do not normally swim about actively attacking large prey. In fact, piranhas are rather inactive fish best described as opportunist feeders. Wild piranhas take not only other fish, but also a variety of invertebrates including snails, crustaceans and insect larvae. One genus, Catoprion, has become specialised as a fin- and scale-eater. Carrion is an important part of their diet, and to some extent the stories of them eating large mammals in particular are probably based on this. In addition to meaty foods, piranhas also eat significant amounts of plant material as well, including algae, seeds and fruits.

Piranhas are relatively inactive for most of the day, and only forage for food at dawn and dusk. They actively avoid brightly lit areas, and are normally found in places with thick vegetation, particularly floating plants.

Social behaviour

Piranhas are schooling fish, but the reasons for this have only recently become clear. For a long time it was assumed that piranhas swam in groups because this allowed them to hunt co-operatively, taking much larger prey than would be possible otherwise.

But recent work has suggested that piranhas school for defensive reasons. During the wet season, when the water level is high and there’s more space to forage and avoid predators, piranhas swim in small groups of just a few individuals. Once the dry season begins and the water level drops, these small groups coalesce into bigger groups of fifty or more individuals. This seems to be a response to the greater danger posed by predators, which like the piranhas are not confined to a steadily shrinking area.

Within the school, piranhas are intensely hierarchical. In the aquarium this can be problematic, especially when kept in groups of less than six specimens. Hierarchical bickering takes the form of chases and biting. In some cases, aggression is so pronounced that adults are best kept singly in their own aquarium.

Broadly speaking, only piranhas from the genus Pygocentrus can be relied upon to form stable groups under home aquarium conditions. Schools of red-bellied piranha Pygocentrus nattereri for example are quite regularly kept in groups of 4-6 specimens. Piranhas from the other genera tend to be schooling fish only when young, and under aquarium conditions at least are too aggressive to be maintained together safely. These piranhas are best kept as a singletons.


In the wild, piranhas spawn among floating plants such as water hyacinth. The eggs are laid on the substrate but the fry hide among the floating plants. Some species at least guard the eggs and fry. A recent increase in piranha attacks on people in Brazil seems to be down to this, with artificial lakes attractive to bathers also turning out to be popular with spawning piranhas.

Most reports of piranhas breeding in aquaria are concerned with Pygocentrus nattereri, probably the most adaptable of all the species commonly traded. Spawning requires a very large aquarium (500-1000 litres). The mating behaviour is a bit rough, and it is important to keep an eye on the fish to make sure they don’t get damaged. Because this species exhibits no reliable differences between the sexes, the usual approach is to rear the piranhas as a school and then remove a pair once they start guarding a nest.

Several hundred to several thousand eggs are laid at any one time. The male does most of the guarding, and will normally drive the female away shortly after spawning. The fry hatch within a couple of days but will not begin feeding until the yolk sac is completely used up. Once this happens, they will take small live foods such as brine shrimp nauplii. The fry are cannibalistic and need to be separated by size as they mature.

Aquarium conditions

Piranhas are not difficult to keep in aquaria provided a few basic things are taken into account. To start with, you need to decide whether you want to keep a school of piranhas or a singleton. All Pygocentrus spp. and a few Serrasalmus will do well in groups of not less than four specimens and ideally at least six specimens. Despite being social fish in the wild, when kept as twos or threes they are apt to be aggressive towards one another, and sooner or later the group gets whittled down to just the one dominant specimen. On the other hand, adult Catoprion, Pristobrycon, Pygopristis and most Serrasalmus will not coexist under aquarium conditions no matter what, and so need to be kept alone.

As with any other large predator these fish need a lot of space and generous filtration. A good rule of thumb is to allow not less than 75 litres per piranha, so a 450 litre aquarium would be the minimum recommended tank for a group of six red-bellied piranhas. Use a filter that provides not less than six times the volume of the tank in turnover per hour. In other words, for a 450 litre aquarium, install a filtration system with a turnover of 2700 litres per hour.

Since they are fish of the weedy riverbanks rather than open water, filling the tank with lots of vegetation is a very good idea. On the other hand, piranhas don’t like bright light. For this reason, going with plastic plants is generally the best idea. Choose varieties that are tall enough to trail across the top of the tank and so produce some shade. If you must go with live plants, use fast-growing species like Vallisneria that will quickly cover the top of the tank and are able to put up with a certain amount of damage should the piranhas feel like eating them.

Water chemistry is generally unimportant. In the wild piranhas are found in slightly acidic to neutral, fairly soft water but the hard, alkaline water of Southern England appears to cause piranhas no particular problems. Water quality, on the other hand, is important. As well as adequate filtration, weekly 50% water changes are essential, and it is also important not to overfeed these fish. Piranha are sensitive to low oxygen concentrations. Do not let the aquarium get too warm (aim for 24-26 degrees Celsius) and if necessary provide additional aeration in summer. Never keep piranhas in overstocked tanks.

Piranhas are obviously not community fish, but neither should different piranha species be mixed because the larger and stronger species will bully (and potentially kill) the weaker one. While some aquarists have kept them with large, armoured catfish such as plecs, this isn’t something to be relied upon and is not recommended.
It should go without saying that piranhas are potentially dangerous animals. They can and do bite their keepers when alarmed. Take appropriate precautions while working in the tank, for example by using a large net to keep the fish in one corner of the tank. Piranhas are especially dangerous if they jump out of the tank, as they often do when scared. While thrashing about it is very easy to end up with the piranha biting the fishkeeper. Under such circumstances, it is important to use long-handled nets and thick, wet cloths to secure and the move the fish safely back to the aquarium.


One of the most common misunderstandings about piranhas is that they will only eat live foods, particularly fish. In fact they are a good deal more omnivorous than this, and in the aquarium accept practically all meaty foods as well as some processed foods. Whitebait, whole prawns, squid, and mussels are eagerly enjoyed by adults, while smaller specimens will take small worms and insect larvae as well.

Once settled in, most specimens will accept carnivore pellets as well.
Goldfish and other cyprinids (such as minnows) must never be used as food for piranhas; besides being a likely source of parasites and bacterial infections, cyprinids contain a lot of fat and the enzyme thiaminase that breaks down Vitamin B1 causing problems over the long term. Other fatty foods, such as meat, chicken, cheese and so on should also be avoided.

Given that wild fish eat some plant material, it is a good idea to try and offer your piranhas some green foods as well, either in the form of things like tinned peas and chunked cucumber, or else algae-based flakes and pellets. While it isn’t really practical to give them the sorts of fruits and seeds they would eat in the wild, some species at least will accept dried seeds such as sunflower seeds as an alternative.

As with any other predatory fish, avoid overfeeding. Feed small amounts once a day, and remove any uneaten food after five minutes.

What’s available at Wildwoods

For the aquarists interested in piranhas, a visit to Wildwoods is always worthwhile. Wildwoods also provide a mail-ordering service, so if you’re not able to get to their store in Enfield, consider arranging to have a shipment of fish sent out to you.

Pygocentrus nattereri ‘Super Red’

Compared with the standard red-bellied piranha, the ‘Super Red’ has a stronger pattern of dark spots on the flanks and the red colouration extends further across the face and belly. It gets to about 30 cm in captivity. A widespread and adaptable species, the ‘Super Red’ is a good piranha for the home aquarium, particularly when kept in a reasonably large group of no less than four specimens and preferably at least six specimens.

Pygopristis denticulata ‘Orinoco’ Lobetoothed piranha

At first glance this species resembles the red-bellied piranha but it can be immediately distinguished by its unusual teeth. It also has red fins that offset its silvery body marked with numerous small black spots. This is one of the smallest of the true piranhas, getting to a maximum size in the wild of about 20 cm, and usually a bit less in the aquarium. As well as all the usual piranha foods, it also takes scales and fins from large and slow-moving fishes. May be kept alone or as a group of six or more specimens. An interesting species, but rare in the wild, and very infrequently traded.

Pristobrycon striolatus

Another small piranha and one of the most omnivorous, wild fish seem to feed primarily on seeds and to a lesser degree the fins of other fish. A very beautiful species that is silvery in colour with a reddish region from the lower jaw back as far as the anal fin. The leading edges of the tail fin are dark grey. Apparently a solitary fish in the wild, this species does well kept as a single specimen in its own aquarium.

Serrasalmus humeralis ‘Xingu’ Pirambeba

This is a medium-sized species distinguished by a elongate black band above the pectoral fin. Its lower jaw and gill covers are orange-yellow while the rest of the body is silvery. This one of the more completely predatory piranhas, feeding primarily on small fish and various benthic invertebrates. Basic care is similar to the red-bellied piranha, and known to do well kept in reasonably large groups.

Serrasalmus sp. ‘Black Guyana’

Apparently a variety of Serrasalmus rhombeus, one of the largest piranhas, and so may reach anything up to 45 cm in length though around 30 cm is more probable. Compared with other piranhas, Serrasalmus rhombeus grow relatively slowly, and it can be assumed that Serrasalmus sp. ‘Black Guyana’ is similar. Even so, while basic care is similar to that required by the red-bellied piranha, the aquarist will need to provide quarters appropriate to the adult size of this fish. Colouration in Serrasalmus rhombeus is variable depending on where the fish is found, ranging from smoky silver to matt black. Serrasalmus sp. ‘Black Guyana’ is, as its name suggests, one of the darker varieties. The trailing edge of the tail fin is transparent. Serrasalmus rhombeus are opportunistic carnivores that eat a wide variety of prey animals including mammals and lizards along with the usual fishes and invertebrates. While juveniles are shy and live in loose schools, adults are more or less solitary and highly aggressive towards one another when kept in home aquaria. As such, this species is best kept alone in its own aquarium.

Serrasalmus gibbus Gold Piranha

Another medium-sized species noted for the bright gold colouration that covers the face and belly. The trailing edge of the tail fin is dark grey. Maximum size is about 20 cm, but aquarium specimens tend to be a bit smaller. In good condition this is a spectacular, adaptable fish that makes an excellent subject for the home aquarium. Requires similar conditions to the red-bellied piranha, but because it is somewhat aggressive it is normally kept as a singleton.

Serrasalmus marginatus

One of the most beautiful piranhas. Rather similar to Serrasalmus spilopleura in looks, but a bit smaller, up to around 22 cm. The body is distinctly rhomboid in shape, silver in colour and covered in small black spots. The fins are quite extravagantly coloured; the dorsal, adipose, and anal fins are yellow tipped with red, and the tail fin deeply forked, yellow in colour and marked with black on the leading edge. This species is a solitary opportunist that feeds extensively on the scales and fins of other fish. It is unusual among piranhas in being territorial rather than migratory. It is extremely aggressive and cannot easily be kept in groups, though it has been spawned in public aquaria and ponds.

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