Pacu Fish Care Guide: Size, Tank, Diet & Piranha Differences

Advice by Jan, keeping fish since 1995 · Updated September 2026

Red-bellied Pacu (Piaractus brachypomus) in an aquarium

Red-bellied Pacu showing its deep body shape

Pacu fish photographed in a home aquarium

Quick answer: “Pacu” is a common name used for several large South American serrasalmid fishes rather than one single species. This guide focuses mainly on the Red-bellied Pacu, Piaractus brachypomus, the species shown in most of our photographs, while also explaining where the much larger Black Pacu, Colossoma macropomum, differs. Red-bellied Pacus can reach extraordinary adult sizes, are powerful swimmers, produce a heavy bioload and should not be treated as normal community-aquarium fish. Their adult housing requirement is measured in thousands rather than hundreds of litres if they are to be kept properly for life.

Main species coveredPiaractus brachypomus (Red-bellied Pacu)

FamilySerrasalmidae — the same family that includes piranhas

Published maximum sizeRed-bellied Pacu: 88 cm; Black Pacu: 108 cm

TemperatureAbout 23-28°C (73-82°F) for Red-bellied Pacu

pHRoughly 4.8-7.5 for Red-bellied Pacu

DietPlant-heavy omnivore; fruits, seeds, nuts and quality prepared foods

Adult tankVery large custom aquarium; footprint matters as much as volume

LifespanA long-term commitment; FishBase reports a maximum age of 28 years for P. brachypomus

What Is a Pacu?

The name Pacu is applied to several related South American fishes in the family Serrasalmidae. In the aquarium trade, two species are especially important: the Red-bellied Pacu (Piaractus brachypomus) and the Black Pacu or Tambaqui (Colossoma macropomum). They are related to piranhas, but they are not piranhas and their feeding ecology, teeth and adult size differ substantially.

This distinction matters because the word “Pacu” is often used too loosely. The two species are both enormous by aquarium standards, but the Black Pacu can become larger still. FishBase records a maximum published total length of 88 cm (34.6 in) and weight of 25 kg (55 lb) for P. brachypomus, while C. macropomum reaches a published maximum of 108 cm (42.5 in) and 40 kg (88 lb).

Feature	Red-bellied Pacu	Black Pacu / Tambaqui
Scientific name	Piaractus brachypomus	Colossoma macropomum
Published maximum length	88 cm	108 cm
Published maximum weight	25 kg	40 kg
Native region	Amazon and Orinoco basins	Amazon and Orinoco basins
Typical wild diet	Fruits, seeds and nuts, plus opportunistic animal foods	Fruits, grains and plant material, with seasonal omnivory

From our experience at Aqua-Fish.Net

One of the first fish I ever kept was a Red-bellied Pacu. Juveniles are attractive, active and easy to fall in love with, which is exactly why so many people buy them before realizing what the adult fish becomes. Over the years we have repeatedly seen owners searching for solutions only after their Pacu has outgrown an ordinary aquarium. With this species, planning for the adult fish before buying the juvenile is not optional.

Pacu vs. Red-bellied Piranha: How to Tell the Difference

Young Red-bellied Pacus can look surprisingly similar to Red-bellied Piranhas. That resemblance is not accidental: juvenile Piaractus brachypomus are reported to mimic the coloration of Pygocentrus nattereri, which may help discourage predators.

The mouth is one of the most useful clues. A Red-bellied Piranha has a stronger projecting lower jaw and sharp, triangular cutting teeth. Pacus have a less predatory jaw profile and broader crushing teeth adapted for fruits, nuts and seeds. Those Pacu teeth can look almost human from a distance, but they are extremely powerful and should be treated with respect.

Red-bellied Piranha

Notice the pronounced lower jaw and cutting profile of the mouth.

Red-bellied Pacu

The juvenile coloration can be similar, but the head and dentition differ.

Pacu are usually peaceful, but their bite is not harmless. FishBase notes that Piaractus brachypomus possesses powerful dentition capable of causing serious injury. Do not hand-feed a large Pacu and take care during aquarium maintenance.

Pacu in the Wild

Red-bellied Pacus are native to the Amazon and Orinoco systems of South America. They use large river channels and floodplains and move into flooded forests during the wet season. This seasonal access to flooded forest is important to their natural feeding ecology.

Wild Red-bellied Pacus are mainly plant-eaters and frugivores, consuming fruits, nuts and seeds, but they are opportunistic rather than strict herbivores and also take insects, zooplankton and small fish. USGS describes them as important seed dispersers in floodplain forests. The Black Pacu has a similarly strong connection with flooded forest and feeds heavily on fruits and grains when those foods are available.

Why this matters in an aquarium: a Pacu is not a “vegetarian piranha” that can simply be fed lettuce. Its natural diet is broad, seasonally varied and nutritionally richer than a single vegetable or one type of pellet.

How Big Do Pacu Fish Get?

This is the single most important fact to understand before buying one. Pet shops commonly sell Pacus when they are only a few centimetres long, but Red-bellied Pacus can grow into fish approaching a metre in length. FishBase reports a maximum of 88 cm for Piaractus brachypomus, and the Black Pacu is even larger, with a published maximum of 108 cm.

Growth can be rapid when juveniles are well fed. A small aquarium may therefore appear adequate for a short time and become unsuitable very quickly. The idea that a fish will simply “grow to the size of its tank” is a myth; cramped housing restricts movement and water quality rather than safely controlling adult size.

How long do Pacu fish live?

Pacu should be regarded as a long-term commitment. FishBase reports a maximum age of 28 years for Piaractus brachypomus. Captive lifespan varies with species and conditions, but this is clearly not a fish to buy on the assumption that it will be a short-lived aquarium resident.

Pacu Tank Size and Aquarium Requirements

The old version of this guide suggested about 250 US gallons (roughly 946 litres). That is already an enormous aquarium by normal household standards, but for a fully grown Red-bellied Pacu it is still not a generous lifetime target.

Think in dimensions, not just litres. Specialist species references recommend a footprint around 300 × 90 cm (118 × 36 in) for an adult Red-bellied Pacu, with an overall aquarium around 300 × 90 × 90 cm — roughly 2,400-2,550 litres — and preferably larger. The exact number is less important than the principle: an adult Pacu needs a custom-sized enclosure with enough length and width to turn and swim naturally.

For that reason, it is fair to ask whether Red-bellied Pacus should be considered ordinary home-aquarium fish at all. Juveniles can be grown temporarily in smaller aquaria, but only if a realistic adult home already exists. A public aquarium, very large custom indoor system, or properly designed tropical pond may be more realistic for a mature specimen than a conventional fish tank.

Keep the interior open and simple. Adult Pacus are active, powerful fish that can move decorations and damage exposed equipment. Large pieces of driftwood or smooth rocks can be used if they are secure, but avoid cluttering valuable swimming space. Protect heaters and thermometers; a sump, external heater arrangement or other equipment outside the main display is safer than leaving fragile glass equipment where a large fish can strike it.

Filtration must be scaled to the animal. Large Pacus produce large amounts of waste, so strong biological and mechanical filtration is essential. Sump systems and even appropriately configured pond filtration can make more sense than ordinary hobby filters. Perform regular substantial water changes and use measured ammonia, nitrite and nitrate values to guide maintenance.

Water parameters for Red-bellied Pacu

Parameter	Practical range
Temperature	23-28°C (73-82°F)
pH	Approximately 4.8-7.5
Hardness	Soft to moderately hard; approximately 0-15 dGH
Ammonia / nitrite	0

Stability is more important than chasing an exact pH. Because warm water carries less dissolved oxygen, very large fish benefit from strong circulation and aeration, especially in heavily stocked or warm systems.

What Do Pacu Fish Eat?

Red-bellied Pacus are best described as plant-heavy omnivores. In nature much of their diet consists of fruits, nuts and seeds, while insects, zooplankton and occasional small fish are taken opportunistically.

In captivity, use a high-quality large-fish pellet or floating stick as a reliable nutritional base and add substantial plant variety. Suitable foods include leafy greens, peas, courgette, carrot and pieces of fruits such as apple or grapes. Variety is more useful than feeding one favourite item every day.

Foods such as shrimp and suitable invertebrate items can be offered occasionally. We would not make chicken, mammalian meat or feeder goldfish routine foods. Feeder fish can introduce parasites and disease, while animal flesh does not reflect the plant-rich natural diet of the species.

Food type	Examples	How to use it
Staple food	Quality large-fish pellets or floating sticks	Use as the reliable nutritional base.
Vegetables	Peas, courgette, carrot and leafy greens	Offer regularly and vary the selection.
Fruit	Apple, grapes and other suitable fruits	Use as part of a varied plant-rich diet rather than as the only plant food.
Occasional animal foods	Shrimp and suitable invertebrate foods	Offer occasionally rather than as the dietary base.
Best avoided as routine foods	Feeder goldfish, chicken and mammalian meat	These are unnecessary and do not match the species' plant-rich natural diet.

Red-bellied Pacus swimming together in an aquarium

Red-bellied Pacus are energetic feeders and powerful swimmers.

Pacu Behavior and Tank Mates

Red-bellied Pacus are generally much more peaceful than piranhas, but “peaceful” does not mean easy to mix with other aquarium fish. Small fish may be eaten, while shy or sedentary fish can be stressed by the Pacu's constant movement. A community containing adult Pacus would need to be exceptionally large.

Juveniles are social and may shoal, while mature fish can become more solitary. Keeping several adults together therefore multiplies an already extreme space requirement. If tank mates are attempted, choose robust fish too large to swallow and able to tolerate similar water conditions. Always have a backup plan because compatibility can change as the Pacu grows.

Jan's observation

My Red-bellied Pacus were lively fish. After feeding they would sometimes chase one another around the tank almost as if playing tag. Newly introduced Pacus could be skittish around sudden movement, but once settled they were engaging fish to watch. Their speed and physical power are much easier to appreciate once the fish becomes large.

Breeding Pacu Fish

Breeding Red-bellied Pacu is not a realistic project for the normal home aquarium. In nature they undertake seasonal spawning migrations associated with the flood cycle. USGS describes adults moving from marginal lagoons and ponds and migrating upstream to spawn, with larvae later carried downstream toward floodplain habitats.

Commercial Pacu breeding is carried out in aquaculture facilities, and reproduction is commonly induced using hormones. Specialist aquarium references note that successful reproduction has not become an ordinary hobby procedure. This is very different from breeding many cichlids, where a compatible pair may spawn readily in a large home aquarium.

For that reason, the former advice to separate parents from thousands of eggs has been removed from this guide: it oversimplified a reproductive process that is normally managed under aquaculture conditions rather than in domestic aquaria.

Why Pacu Can Be Such Memorable Fish

Pacus can be remarkable animals to keep because they are active, responsive and visually impressive. That same appeal, however, is the reason juveniles are so often purchased impulsively. The responsible way to appreciate them is to treat their adult size as the starting point of the decision, not as a problem to solve later.

Never release a Pacu into local waters. USGS records Pacu captures far outside their native range and identifies aquarium release as a likely pathway in many cases. If a fish becomes too large, arrange rehoming through a specialist keeper, large aquarium facility or responsible aquatic organisation instead of releasing it.

Pacu Fish Pictures

Pacu Fish Frequently Asked Questions

How big do Red-bellied Pacu get?

FishBase reports a maximum published length of 88 cm (34.6 in) for Piaractus brachypomus. Even captive fish that remain smaller than the published maximum can easily outgrow ordinary home aquaria.

How big do Black Pacu get?

Colossoma macropomum is even larger. FishBase records a maximum published length of 108 cm and weight of 40 kg.

What tank size does a Pacu need?

For an adult Red-bellied Pacu, specialist references recommend a base around 300 × 90 cm, with an aquarium of roughly 2,400-2,550 litres or larger. Juveniles can be kept temporarily in smaller tanks, but only with a realistic adult housing plan.

Are Pacu piranhas?

No. Pacus and piranhas are related members of the family Serrasalmidae, but they differ in dentition, feeding ecology and behavior. Juvenile Red-bellied Pacus can resemble Red-bellied Piranhas.

Do Pacu have human-like teeth?

Pacu have broad crushing teeth that can look surprisingly human-like, but they are adapted for processing tough plant foods such as fruits, nuts and seeds. A large Pacu can still inflict a serious bite.

What do Pacu fish eat?

Red-bellied Pacus are plant-heavy omnivores. Use quality pellets or floating sticks together with varied vegetables and some fruit, with occasional suitable invertebrate foods.

Can Pacu live with other fish?

Yes, but only in a truly enormous aquarium. Small fish may be eaten and shy species may be stressed by Pacu activity. Compatible tank mates must be robust, too large to swallow and suited to the same water conditions.

Are Pacu fish aggressive?

Red-bellied Pacus are generally peaceful for such large fish and are far less predatory than piranhas. However, adults may chase or nip other fish when space is limited, and any fish small enough to swallow can become food. Their powerful jaws can also cause a serious accidental bite even though Pacus are not normally aggressive toward people.

Do Pacu eat other fish?

Yes, they can. Red-bellied Pacus have a strongly plant-based natural diet, but they are opportunistic omnivores rather than strict herbivores. Small fish may be eaten, especially when they fit easily into the Pacu's mouth, which is one reason small community fish are unsuitable tank mates.

What is the difference between a Pacu and an Oscar fish?

Pacus and Oscars are very different South American fishes. Red-bellied Pacus can grow much larger, have a strongly plant-based omnivorous diet and eventually require an enormous aquarium measured in thousands of litres. Oscars are large cichlids, but remain considerably smaller and have different feeding, territorial and housing requirements. See our Oscar Fish Care Guide for detailed Oscar care information.

How long do Pacu fish live?

They are potentially very long-lived fish. FishBase reports a maximum age of 28 years for Red-bellied Pacu, so they should be regarded as a decades-long commitment.

Can Pacu breed in a home aquarium?

Breeding is not regarded as a normal home-aquarium procedure. In nature Pacus are seasonal migratory spawners, while commercial aquaculture commonly relies on induced spawning.

Can Pacu be kept in a garden pond?

Only where temperature, water quality, depth, filtration and escape prevention are suitable for a tropical fish of enormous adult size. In cool climates an ordinary outdoor pond is not appropriate, and Pacu must never be released into natural waterways.

For species-specific information, see our Black Pacu - Colossoma macropomum profile and Red-bellied Pacu - Piaractus brachypomus profile.

References and further reading

This guide combines long-term aquarium-keeping experience with established biological and specialist fishkeeping references. See FishBase: Piaractus brachypomus, FishBase: Colossoma macropomum, Seriously Fish: Piaractus brachypomus, and USGS Nonindigenous Aquatic Species: Red-bellied Pacu.