Keeping Piranha

There are five genera of fish currently recognised under the broad definition
of piranha species. These are all fish which fall within the taxonomic subfamily
of Serrasalminae. This article is concerned with the two genera most widely
recognised and sold as piranha: Serrasalmus and Pygocentrus.

The two
genera themselves are relatively easy to tell apart. Pygocentrus species are the
classic round-shaped, bulldog-jawed species. The Pygocentrus genus is generally
held to be made up of only three recognised species; P. piraya, P. cariba (often
referred to incorrectly as P. notatus) and P. nattereri. The last of these is,
of course, the famous red bellied piranha and is the species most commonly seen
in nature documentaries and horror films. The glaring inaccuracies and
sensationalism of some documentaries can make it difficult to tell these two
types of film apart, highlighting the degree to which these fascinating fish are
misunderstood.

These overly dramatic descriptions of Pygocentrus species do have some basis
in fact. In the wild, pygos live in large groups and are capable of taking on
prey much larger than themselves, due to their distinctive “feeding frenzy”.
Creatures as large as cattle or horses can be overwhelmed by sheer weight of
numbers, with hundreds of fish ripping small chunks out the prey’s flesh until
it succumbs. In reality, piranha are unlikely to attack such large animals
(including humans) unless the animal is bleeding or is otherwise showing signs
of injury. Indeed, the Amazonian Indians that inhabit the shores of the rivers
in which piranhas are found show no hesitation in swimming, washing clothes and
bathing in piranha-infested rivers, knowing that piranha will avoid contact
wherever possible. This behaviour is replicated in the aquarium and most piranha
will take cover when tank maintenance is being carried out. Care is always
advised, however, as their teeth are incredibly sharp!

There is some
debate as to whether the Pygocentrus genus actually does contain only three
species, as there are several other contenders for membership. The first of
these, Pygocentrus palometa, was first described by the ichthyologist A.
Valenciennes in 1849. It is listed as occurring in the lower Rio Orinoco,
Venezuela but little further information is available. Although the scientific
name is still listed as valid, it is highly likely that this P. palometa is
actually a synonym of Myleus schomburgkii, a member of the silver dollar family.
The second species, held by many to be a valid Pygocentrus, is P. ternetzi. This
issue is yet to be settled conclusively but current scientific thinking classes
this fish as a yellow colour form of P. nattereri. Finally, some ichthyologists
believe that species most commonly sold as the red belly is in fact a species
known as P. altus and that the true P. nattereri species is never exported from
what is believed to be its relatively small geographic range. This theory has
not found a great deal of acceptance amongst piranha enthusiasts, with most
believing that P. altus is simply a junior synonym of P. nattereri. In any
event, these misunderstandings highlight the need for further study of this
group of fish.

The Serrasalmus genus, in contrast to Pygocentrus,
contains a larger number of species but the majority of these are solitary in
the wild. Some of the species may form loose groupings but it is not recommended
that this is attempted in the limited confines of the aquarium. The Serrasalmus
species most commonly seen in the hobby are S. rhombeus, S. brandti, S.
elongatus, S. geryi, S. sanchezi and S. maculatus / spilopleura. Less commonly
seen are S. manueli, S. altuvei, S. hollandi, S. irritans, S. compressus, and S.
marginatus. The remaining members of the genus rarely, if ever, make it into the
hobby in the UK. The majority of the Serrasalmus species are diamond shaped,
some quite prominently so, although there are some species that are more similar
in shape to Pygocentrus (S. maculatus, S. manueli and S. humeralis).

Even more so than the Pygocentrus genus, the Serrasalmus genus has been
subject to regular taxonomic reorganisation. Indeed, all the Pygocentrus species
were originally placed within Serrasalmus, along with many other would-be
piranha species that are now habituated into other genera. Some Serrasalmus
species are still regularly misidentified; the most common of these is S.
rhombeus, which is still often referred to as S. niger. This is, of course,
where this species gets one of its common names, the Black Piranha.
Additionally, it can be difficult to tell some of the Serrasalmus species apart,
especially when juvenile. The compressus group (S. irritans, s. compressus , and
s. marginatus) is a good example of this and also, distinguishing juvenile S.
rhombeus and S. sanchezi can be a challenge.

The solitary nature of the
Serrasalmus genus dictates a different feeding method in the wild from
Pygocentrus. Many serras are stealth predators, lurking under cover and making
lightning attacks on prey. They are also opportunistic feeders and some species
trail along behind packs of pygos and finish off any scraps that the group leave
behind. Other serra species are fin and scale eaters and do not actually kill
the fish on which they prey. One theory for this is that the serras are somehow
programmed not to kill their prey and thus leave the food source intact for
another meal.

Analysis of stomach contents of captured serras (and also
pygos) shows nuts, fruit and other vegetable matter, leading some to conclude
that piranhas may be more omnivorous than is generally believed. Further study
is required, as another explanation of this could simply be that, when caught,
the piranha had recently eaten a fish which in turn had eaten the vegetable
matter found.

Piranha in the Aquarium

Piranhas occur
across a fairly broad geographic range in South America, from Colombia and
Venezuela in the north, down to Southern Brazil and Paraguay. As a result, there
is a broad range of water conditions that these fish will tolerate. Our species
profiles give more information on individual fish but, in general, a piranha
aquarium should have a pH of around 6.8, a temperature of around 78°F (26°C) and
a water hardness level of not more than 20°H.

As one might expect, there
is no definitive tank size for keeping piranha. Keeping pygos as a group will
require a large tank, as the adult fish are capable of attaining 14″ or more
depending on the species. Realistically, a 6′ x 2′ x 2′ (180cm x 60cm x 60cm)
will be needed for a group of 6 – 8 fish. Obviously, juvenile fish can be grown
on in smaller tanks but there should always be consideration of the long term
accommodation plans.

Serrasalmus, as solitary fish can obviously kept in
smaller tanks but, again, the adult size of the species you wish to keep should
be researched. S. manueli and S. rhombeus are both capable of attaining sizes of
up to 24″ (60cm), albeit over a period of many years. In general, an aquarium of
around 48″ x 15″ x 15″ (120cm x 37.5cm x 37.5cm) would do the majority of the
serra species for a long time, if not for life.

Equipment and
decor

Piranha are much less confident fish than they are generally
believed to be. As such, dimmed or reduced lighting in the aquarium will
encourage these fish to emerge from their hiding places. This level of lighting
can pose a challenge in planted tanks, as maintaining both a piranha and healthy
plant growth can be difficult. It is recommended that hardy plants with low
light requirements are used. Anubias and cryptocoryne species make good choices
for this.

Despite the difficulty of growing plants in dimmed conditions,
it is recommended that piranha tanks are well planted as the fish will
appreciate the cover. If you are not striving for a completely natural aquarium
then artificial plants could also be used and even interspersed with real plants
to disguise their fakery. Bogwood, branches and rockwork can all be considered
too. Care should be taken to ensure any heavy decor is well secured, as piranha
are powerful fish and could potentially crack the tank if they knocked a large
rock or similar item into the aquarium wall.

The more hiding places that
are offered, the more the fish is likely to feel secure enough to venture into
open water. This is especially true of serras as they lack the “safety in
numbers” feeling that pygos tend to have, living as a part of a group.

Filtration is an important consideration in a piranha aquarium. Piranhas
are large messy fish, fed on a high protein diet and as a result, the tank water
can become very easily polluted. It is therefore imperative that the filter
selected is up to the task. Internal filters are not really suitable for piranha
tanks as they do not have an adequate capacity to deal with the levels of waste.
Ideally you should use an external filter (or filters) and aim to turn over the
tank volume five or six times per hour, for example in an aquarium of 250
litres, the output of the filter(s) should be around 1500 litres per hour (lph).

Even with an abundance of filtration, a dedicated water maintenance regime
should still be followed. It is recommended that weekly water changes of 20% are
carried out. Nitrates should be monitored on a regular basis as, in addition to
being generally undesirable in an aquarium, they also inhibit grow rates. This
is true for all fish but piranha growth seems to be particularly affected by
higher levels of nitrate. Nitrates should be kept as low as possible, ideally
nil but this can be impractical to maintain, especially in areas that already
have raised nitrate levels in their tap water.

In addition to a filter,
it can be a good idea to add a power head or other water pump to a piranha
aquarium. This creates a strong water flow in the aquarium which in turn, forces
the fish to keep active, swimming against the current. This has the effect of
boosting the metabolism of the fish, meaning it should always be ready to feed.
Many experienced piranha keepers use this trick, combined with an increase in
water temperature, to encourage difficult fish to begin feeding.

Tank
mates

Piranha and tank mates can be a very hazardous combination! As
a general rule, all of the Pygocentrus species will live together, unless there
are significant size differences between the fish. If some of the tank
inhabitants are much smaller, then there is a very real chance they will be
eaten. However, this process can also work in reverse because, if there are a
number of smaller pygos in a tank and one or two large fish, the smaller fish
can sometimes “gang up” on the large fish and kill it, due to the perceived
threat of having the large fish in the same territory as the smaller ones.

Many piranha keepers claim that they successfully keep other species
with their piranha, such as large armoured catfish like plecos or very small
tetras, which the piranha seem to ignore. In some instances, this arrangement
can work successfully for some time but usually ends in disaster. The bottom
line is that you shouldn’t keep any fish with a piranha, unless you are prepared
to risk losing it.

As mentioned earlier, some of the Serrasalmus species
can be found in groups in the wild, the main species being S. maculatus, s.
geryi and S. sanchezi. Some enthusiasts have attempted to replicate this in
captivity – with varying degrees of success. Again, this course of action is not
recommended due to the significant possibility that any serra shoal will likely
end up as a single fish, over time.

Feeding

Piranha do not require live fish (or any other form of
live vertebrate) as part of their diet in captivity! The subject of feeding is
one that divides many piranha keepers. Those in the “pro” live fish camp
maintain that feeding live fish replicates natural conditions and behaviour,
while those in the “anti” live feeding camp feel that feeding live fish is
unnecessary and cruel. It is worth pointing out that it is illegal in the UK to
use any live vertebrate (including fish) as food for another animal.

Piranha (and other predatory fish) are programmed to survive. This means
that they will adapt to the available food sources and will not let themselves
starve to death. With perseverance, any piranha will accept dead foods. If you
acquire a fish that refuses to feed then continue to offer food on a daily
basis, removing it after a couple of hours if the fish does not eat it to avoid
polluting the water. Eventually, hunger will take over and the fish will accept
the food offered. Personally, I have kept almost all of the commonly available
piranha species (and a few that are not so common) and all have been converted
to dead foods in this way. Some species or individuals can take several weeks to
begin feeding (my personal record is held by a Serrasalmus gouldingi that took
six weeks) but will invariably give in.

The main diet of piranha should
be meaty foods such as prawn, mussel, squid, whitebait, lancefish and cockles.
Indeed, almost any whitefish or shellfish can be used. The feeding of oily fish
such as salmon and mackerel is not recommended due to the extent to which they
can pollute the water. It is good practice to vary the foods offered as much as
possible, as occasionally piranha can become fixed on one food type if it is
offered continuously, leading to problems when trying to get the fish to accept
something different. Other meat-type foods such as beef heart and chicken can be
fed as an occasional treat but are not really recommended. Piranha (and most
other fish) do not have the physiology to effectively digest animal protein and
this can lead to liver failure if fed regularly.

When juvenile, piranha
may accept live or frozen foods such as bloodworm and brine shrimp but will
almost certainly grow out of this, as the food becomes too small to offer them
much sustenance. Additionally, it can be worth experimenting with dried foods
such as cichlid pellets when the fish are young. Many aquarists report successes
in introducing their baby piranha to such foods, which they continue to eat into
adulthood. The benefit of this is that these types of pellets represent a more
balanced diet and contain vitamins and other nutrients that are not necessarily
present in a mainly fish-based diet. If your piranha will accept these then they
should be incorporated into the variety of foods discussed above.

As
discussed earlier, there is a school of thought that suggests piranha may be
more omnivorous in nature than they are given credit for, so there is no harm in
offering piranha any foods that you would offer to other fish. If they eat them,
then keep feeding them! For fish that refuse to eat anything except meaty foods,
some piranha keepers recommend soaking the prawn or mussel or similar in a
vitamin supplement prior to feeding. One of the most commonly available
supplements is a product called Vitazin. Another means of getting your piranha
to eat a broader diet is to stuff some cichlid pellets (or other dried food)
into the prawn or lance fish etc prior to feeding it. Most piranha eat their
meal in one or two bites and will be unaware of the additional course!

Breeding

Pygocentrus nattereri is the only piranha that breeds
with any frequency in captivity. In fact, the majority of juvenile red bellies
offered for sale in UK aquatic stores will be captive bred. This does not mean,
however, that they are an easy fish for the hobbyist to spawn. Piranha are not
externally sexually dimorphic, meaning that the only way to tell the sexes apart
is when they are actually spawning. Should you wish to attempt spawning red
bellies then it is recommended that you begin with a group of 6 – 8 individuals
in a very large tank with minimum dimensions of 6′ x 2′ x 2′ (150 UK gals),
although bigger is recommended.

Water conditions in the tank should be
fairly soft an acidic and, in fact, there are some reports that spawning may be
induced by a fairly rapid drop in pH. This can be achieved in the aquarium by
carrying out large water changes with water that has been filtered through peat.

If the correct conditions can be recreated, a pair will hopefully form.
The fish go very dark when in breeding condition. The pair will split off from
the group and excavate a pit in which spawning occurs. The pair will become
aggressive to the rest of their tank mates at this point, hence the reason as
large an aquarium as possible is recommended. It is not generally a good idea to
remove either the pair, or the other fish from the aquarium, as re-integrating
the group later is likely to pose a major challenge.

Hundreds of eggs
are produced in each spawning and the adult fish do not demonstrate strong
parental behaviour. Eggs hatch in around 48 hours and the fry can be fed on
newly hatched brine shrimp and other tiny foods once they are free-swimming,
approximately three days later. Fry are cannibalistic and the larger fry will
inevitably eat the smaller ones. Unless there is sufficient space to house all
the fry, this is an effective way of controlling the size of the brood and
ensuring only strong fish survive.

Fry will initially be ignored by the
other piranha in the tank as they are much too small to be considered food but
if the intention is to grow on some of the fry, then they should be removed to
their own aquarium at this stage. This aquarium should be filled using water
from the main tank to ensure the fry do not die of shock when transferred. If
good water quality and adequate feeding is maintained, then growth rates will be
fairly rapid to the stage where the juvenile fish can be sold on.

The
only other species recorded as breeding in captivity is Serrasalmus maculatus
but few details are available. It is assumed that the other Pygocentrus species
spawn in a broadly similar fashion to that described here.

Summary

Piranha are fascinating, yet
largely misunderstood, fish. They have a awesome reputation to which they rarely
live up, but are still believed by many to be the most fearsome fish in the
world.