Pacu fish (Colossoma macropomum)

Pacu fish (lat. Colossoma macropomum) is a freshwater ray-finned aquarium fishes kind from the subfamily Serrasalminae, Characidae family, or (according to another classification) from Serrasalmidae family, the only kind of Colossoma genus. It is one of the piranha families known for its blood lust. But unlike its “cousins” and despite a strong and massive body, this fish prefers plant food and belongs to the herbivorous class.

Habitat in the wild

Colossoma macropomum is from the Colossoma genus, which is widespread in South America. The Characidae belongs to this genus. One of the popular tank fish – neon tetra, belongs to this kind.

Colossoma macropomum was described by the famous biologist Georges Cuvier in 1816 for the first time. Currently, the family’s existing classification is quite complicated, and it is quite controversial even now.

Once large Colossoma species appeared in tanks, they started to call them pacu.

The word has Brazil-Indian origin. In Amazonia, they use this name for Metynnis, Mylossoma, and Myleus kind representatives, which are smaller than Colossoma macropomum, which is known more as tambaqui in its homeland.

The fish body shape resembles the piranha’s one. Despite the significant difference in size, black pacu and piranha have the same number of teeth. In contrast, these have a rather different shape which indicates the difference in their dietary habits.

While piranha has triangle-shaped pointed teeth and quite protrude mandible, pacu has a significantly protrude maxilla with its teeth having a square shape somehow resembling the human’s ones.

Strong teeth allow the fish to crack nutshells which other herbivorous fishes can’t do. Instead of piercing the prey with its teeth and tear it apart, the fish bites it to the bones and tears parts from it.

It is the largest Characiformes representative in South America. This is the second-largest fish after arapaima in the Amazon River basin. Some species can grow to be longer than 1 meter and weigh more than 30 kg. At that 10 % of their mass is fat.

The fish inhabits the Amazon and Orinoco river basins, except large river headstreams with black and milk water. The high-water season there is between mid-December and mid-June.

When rivers’ water level starts rising quickly, adult fish species gather in schools and swim to spawn. We still don’t know the exact location of their breeding ground, but it is likely to be located along the flooded streambeds. After the spawning is over, the schools break up, and the fish migrate to bottomland forests with black water, where they feed on fruit and seeds.

During this period, the fish accumulate a significant amount of their fat which they’ll need in the future when the water level becomes low and the food amount decreases.

Depending on the bottomland type and water level, the fish stays there from four to seven months. When the water level drops, most of them get back to their initial habitats, and some stay in bottomland forests. Their juveniles stay in blackwaters till the time they become reproductive. As a rule, pacu prefers solitary.

Juveniles feed on phytoplankton, insects, snails. The adults are mainly herbivorous, and they feed on fruits and other plant food.

Black pacu gather fruit and seeds after they are ripe and fall in the water. The fish eagerly eats large seeds of both dry and juicy fruit. For example, rubber tree seeds (Hevea spruceana, Euphorbiaceae) compose about 58% of the total fruit amount they eat during this time. The fruit is like a capsule that blows after it gets ripe, and its seeds fly in all directions. They are about 4 cm large and covered with a quite thick coating that only pacu’s strong teeth can crack. The fish gather under the trees waiting for the seeds to fall in the water. The second major component of the fish diet is the large fruit of Astrocaryum jauari palm tree that has the same thick coating.

In the wild, pacu fish also act as seed spreaders. They swim up to several kilometers away from the place where they swallowed the seeds. At that, the seeds get spread through the territory proper for them to grow – along the flood-plain of the rivers.

It appears that pacu, together with terrestrial animals, plays a key role in spreading seeds in the Amazon River basin. Large-sized species are the most efficient in this case since they eat more and swim farther. However, each year the number of such species in the wild decreases. Excessive fishing trade has thinned out the number of some populations at about 90%.

Considering the fish’s role in spreading tree seeds, we can assume that its extinction may lead to the tropical rain forest areas shrinkage.

Being a tropical fish, it dies in too cold water while it adapts rather well in warm waters where it often becomes an invasive alien species. In the 90s, the kind was introduced in Papua New Guinea island. Local conditions appeared to be so favorable that the fish started to grow the size they could never reach in their habitat.

While in the tropical area of South America, pacu is a favorite object of cultivation due to its astonishing growth speed and high eating qualities. The fish is famous for its soft and sweetish taste. During the first year, the juveniles grow to weigh more than 1 kg.

Difficulties in keeping

There are a lot of different fishes with pacu names. However, quite often, their classification is rather confusing. It is interesting, not demanding, and easy to care for and feed fish, which has its kind of intelligence.

Sounds like an ideal aquarium fish, doesn’t it?

However, the biggest problem in care is that it grows fast to become a huge fish, and rather quickly, it outgrows even very spacious tanks. The problem is that on the market, the fish is quite often sold as juveniles and then mistaken for piranha.

Although these fishes really look alike, fish is less aggressive and not that predatory since it feeds on plant food. But it doesn’t eliminate the fact that pacu will eat any small fish in a tank without any doubt.

It’s definitely not a fish for everyone.

It is a very large fish, and it’s better to be kept in very spacious tanks – not everyone can afford to have such a fish at home, although it’s rather not demanding and easy to care about.

It’s not very strict to water parameters. They shouldn’t be too extreme, and the same is about food.

To keep just one juvenile species, you’ll need 300 gallons tank, and for the adult fish – about 650 gallons.

Such a tank requires very thick glass since if fish gets scared, it can break the glass.

Description

Max size is about 1.1 m (3.6 ft), but usually, it’s about 0.7 m (2.3 ft) long, and it weighs up to 32.4 kg (71 lb). Provided with optimal conditions, pacu lifespan in a tank may be up to 25 years long.

Its appearance resembles piranha – the body is high, laterally flattened with big eyes.

The body color varies from black to gray with some variations. About 10% of the body mass is fat. The fish has square human-like teeth that make them incredibly similar to the human ones.

Pacu and piranha juveniles are very much alike. They have dark spots scattered over a silvery-steel body. Such coloring, together with a specific body shape, make the fish a pleasing object to have in a tank.

As the fish grows, its appealing coloring becomes less contrast and changes from whity-brown to almost back. Its anal and pectoral fins are of the same color as the fish body. Water transparency and color affect the fish’s color intensity. In black water dyed with humic acids, such as in Rio-Negro, they have a very dark color, while in white waters, they are much lighter-colored up to having a light-golden body.

Care and keeping in a tank

Scientific NameColossoma macropomum
Common NamePacu fish, paku fish, paco fish, black pacu
Tank size650 gallons and more
TemperamentPeaceful
DietOmnivorous
Temperature75–81 °F (24–27 °C)
pH6.0-8.0
Sizeup to 100 cm (40 in)
Lifespanup to 20 years
Scientific NameColossoma macropomum
FamilySerrasalmidae
Common NamePacu fish, paku fish, paco fish, black pacu
Ease of keepingEasy
Lifespan10 years and more
Tank size1000 liters (220 gallons) and more
TemperamentPeaceful to large fishes
Tank typeCommunity of large fishes
DietHerbivores
Temperature75–81°F (24–27 °C)
pH6-8
Water hardness9–20 dGH
SizeUp to 20 cm (8 in), usual 17 cm (6.69 inch)

Tank size

In petshops, the fish is often sold as a vegetarian piranha. The juveniles are usually about 5-8 cm (2-3 in) large, but the sellers often forget to warn their potential buyers that a small-sized tank will not stop the fish from growing. As a result, these aquarists cannot create proper tank conditions for the fish and let them go to natural water bodies.

The main requirement is a very large tank for adult species from about 1000 liters capacity. If you can afford such a tank – here the difficulties end, since the fish isn’t a demanding one, is not prone to diseases, and eats any food.

The only thing is that they require a very strong filtration because they pollute tank water a lot.

Water parameters

The tank water temperature should be constant about 75–81°F (24–27 °C), dGH to 20°, pH about 6-8 with strong filtration, aeration, and regular water renew. Not for long, the fish will do with water temperature decrease up to 22 °C. The tank water temperature of about 14-17 °C can be considered critical. This is when the fish dies.

The water you add to the tank shouldn’t contain a large amount of solution gas since the fish kind is sensitive to them. They may cause a gaseous embolism and lead to fish death.

Tank setup: decorations and plants

The fish inhabits the middle water layers, and it needs some free space to swim. The best tank decorations are snags and large rocks, don’t even try to put any plants – they are feed for fish.

Fish is a bit timid – any unexpected movement causes a panic attack, and it starts swimming around the tank, hitting the objects and glass walls. If the glass isn’t sick enough, the chances are high that the fish will break it.

Feeding

Fish is omnivorous, and in the wild, it feeds on fruit, corn, insects, snails, spineless species, and carrion. Plant food should prevail in the diet. However, the fish eagerly eats worms, blood worms, tubifex.

But too much protein leads to lipoatrophy quite fast. This is especially true for adult species since, for teens and juveniles, the proportion of protein food can be higher, about 40%.

They eagerly eat pieces of tropical fruit (bananas, pineapples) and vegetables (tomatoes, pumpkin, salad, cabbage, etc.). They can even easily eat watermelon rinds.

Tank mates

Adults are loners, but they aren’t aggressive. However, the juveniles are more troublesome and aggressive. The adults feed on any small fish that they can swallow, but they won’t do any harm to large fishes.

Any middle and large-sized non-aggressive species will do as pacu tank mates, namely, herbivorous catfish (like sailfin pleco). It will pick up food leftovers from the tank bottom. The best tank mates for pacu are fishes of the same size (oscar fish, green terror, jaguar cichlid), or you’d better keep them alone in the tank.

As for the small fishes – they are treated as food, so don’t keep pacu in a tank with them.

Provided with a proper-sized tank and good care, it can become a nice responsive pet. Nevertheless, you should be careful when keeping it in the tank. Although the fish’s diet contains mainly plant food, its strong jaws that can crack very hard seeds and nuts may bring serious danger.

Gender differences: male vs female

Gender dimorphism is quite feeble. Reproductive females that grew in favorable conditions are significantly larger and fatter than males. The abdomen shape is a distinctive feature of pacu females. Males have a more sharp dorsal, their anal fin has pins, and the fish itself is brighter colored than its females.

Breeding

Breeding is impossible in a tank because of the fish size. All species that are on the market are bred in ponds or at farms.

In the wild, usually, the fish is kept alone.

They have a seasonal spawning period that occurs during the rainy season when the water level in rivers rises high and causes water pH and hardness to decrease.

During this time, adult species gather in large schools and migrate deep into the flooded forest, where they spawn in thickly planted areas.

In the breeding industry, they take the fish reproductive products and then incubate the fertilized eggs in special containers.

The females can produce from 50 to 200 thousand eggs. They stick to the substrate or scatter over the bottom. At the temperature of 26—29°C 5-7 days later, the fish larvae start to swim.

Tiny-sized plankton is their start food as well as small-grained high-quality dry food with an obligatory plant component.