Red-Bellied Pacu (Piaractus brachypomus) – Juvenile Size Options

Decide whether you can house a giant pacu

Red-Bellied Pacu, Piaractus brachypomus, is a giant freshwater fish in the family Serrasalmidae, the family containing piranhas and pacus. Pirapitinga and Freshwater Pompano are other established names for the same animal. Its scale, activity and waste production make it an advanced-care choice for a keeper who can provide a purpose-built lifelong system rather than an ordinary community aquarium.

FishBase records a maximum of 88 cm total length. It places the species in the Amazon and Orinoco river basins of South America and also reports it from Argentina. Treat that maximum as the decisive planning figure: an animal approaching this length needs room to swim and turn without meeting the walls, décor or exposed equipment at every movement.

Make the adult commitment first

A juvenile can be housed temporarily in a smaller grow-on aquarium, but it will grow quickly when well fed. The lifelong enclosure, filtration capacity and maintenance routine therefore need to be realistic before the fish is chosen. Delaying the decision until the juvenile looks cramped transfers the risk to an animal that is already difficult to accommodate.

Some gallery images have been digitally enhanced to improve image quality. They show the listed type of animal rather than guaranteeing the exact markings of the individual supplied, so natural colour and pattern can vary.

Choose the dispatch size with the adult system in mind

One purchase quantity supplies one Red-Bellied Pacu of the selected option. The juvenile dispatch sizes are 2–3 cm, 3–4 cm, 5–6 cm, 7–8 cm and 10–15 cm. Use the live size selector to see which option can be chosen when you order rather than relying on static wording elsewhere on the page.

The size you select is the size supplied, not the adult size. These measurements are dispatch grades, so they describe the animal at the point of supply rather than its final potential. A larger juvenile option may reduce the length of an early grow-on stage, but every option leads to the same lifelong adult-space decision.

Sex is guaranteed only when the selected option explicitly states male or female. This Red-Bellied Pacu listing has no sex-guaranteed option.

Use the lifelong dimensions as the pass-or-stop test

Seriously Fish advises at least 300 × 90 × 90 cm and 2,550 litres for one adult, preferably larger. The 300 cm length provides a long swimming run, while the 90 cm front-to-back dimension is part of the requirement rather than an interchangeable amount of water. A taller but narrower aquarium does not replace turning width.

The same profile gives an aquarium range of 23–28°C, pH 4.8–7.5 and 0–15 dGH. These are broad tolerances within which extremes should be avoided. Choose a stable part of the range that matches the conditioned aquarium and make any adjustment gradually.

• Proceed when the adult enclosure, large biological filter and maintenance capacity are credible from the outset.
• Pause when the plan depends on a normal community tank, an uncertain future upgrade or finding another home after growth.
• Recalculate when another giant fish is proposed, because it adds adult space, food and waste rather than merely sharing unused water.

Before placing an order, read the current delivery and Live Arrival Guarantee policy and prepare the receiving aquarium around the selected dispatch grade.

Build the lifelong system around floor space

Start with the lifelong minimum of 300 cm length, 90 cm front-to-back width and 90 cm height, holding at least 2,550 litres, and choose a larger enclosure where practical. All three dimensions matter. Volume alone can conceal a narrow layout that leaves an 88 cm fish with poor turning room, while a generous footprint makes movement and maintenance easier to plan.

Separate the grow-on stage from the final plan

A young pacu may be grown in a smaller aquarium, but that aquarium is a temporary stage rather than a way to reduce adult growth. Decide in advance what measured size, body depth or swimming behaviour will trigger the move. The next enclosure should be operating before that point, so the fish is not left waiting while a large system is ordered, built or matured.

Use the adult dimensions when sizing filtration and access even if the first occupant is small. Seriously Fish notes that adults are too often kept in cramped conditions and that youngsters grow quickly when well fed. Designing around the juvenile’s first feeding load would leave the system progressively less capable as body mass and food intake rise.

Do not trade away footprint for ornamental complexity. This is a very active species, and open water is more useful than dense rockwork or a heavily planted layout. The adult enclosure should make routine observation possible from more than one point, allowing changes in swimming, condition or feeding response to be noticed before they become severe.

Hold water quality with mature heavy-duty filtration

Large biological filtration is essential because a pacu’s food intake and waste rise sharply with growth. A sump-style arrangement is useful where practical because it can keep much of the equipment outside the swimming space while providing accessible mechanical and biological stages. Whatever format is chosen, size it for the adult animal rather than the first juvenile meals.

OATA’s very-large-fish guidance sets zero mg/L ammonia, zero mg/L nitrite and nitrate no more than 20 mg/L above the normal tap-water level for the group. Use those operating limits alongside the pacu’s aquarium range of 23–28°C, pH 4.8–7.5 and 0–15 dGH.

The RSPCA explains why a well-set-up biological filter is vital: fish release toxic ammonia, beneficial bacteria convert it to toxic nitrite, and further bacteria convert nitrite to the safer nitrate. A filter is mature when this biological processing is working consistently, not merely when the equipment has run for a particular number of days.

Use measurements to control the system

Use our aquarium water chemistry guide to interpret the tests, then enter measured values in the water matcher for comparison. Do not chase a preferred-looking value through rapid chemical changes. Stability inside the stated aquarium range is the safer operating plan, provided ammonia and nitrite remain at zero and nitrate is controlled.

Prepare the aquarium before the fish arrives

The receiving aquarium must be heated, biologically mature and tested before the fish arrives. Work through our guide to preparing and maturing an aquarium, then confirm that ammonia and nitrite both read zero mg/L. Check the heater, filter flow and access to conditioned replacement water while there is still time to correct a fault.

Complete the pre-arrival checks

1. Set temperature within 23–28°C and keep it stable.
2. Measure pH and hardness, confirming that they fall within pH 4.8–7.5 and 0–15 dGH.
3. Test ammonia and nitrite and proceed only when both are zero mg/L.
4. Inspect filter inlets, outlets and internal equipment so the open swimming route is not obstructed.
5. Prepare tap-water conditioner and the equipment needed for a partial water change.
6. Leave the aquarium calm and avoid adding several other fish at the same time.

The purpose is to avoid an abrupt temperature or water-chemistry change while moving the animal into clean aquarium water without unnecessary delay. Keep activity around the aquarium low during the settling period and observe swimming and breathing before offering the first meal.

Use the first week to establish a baseline

Record temperature, ammonia, nitrite and the fish’s normal posture each day during the first week. Watch whether it swims in a balanced way, responds to its surroundings and begins accepting appropriately sized food. Clear, bright eyes, intact scales, undamaged fins and appropriate swimming behaviour are useful signs when establishing what is normal for that individual.

Test promptly if the fish hangs at the surface, sits unusually on the bottom, swims erratically, breathes faster or slower than normal, darkens or pales, hides more, or loses interest in food. These changes can accompany deteriorating water or a health problem. Checking the measurable conditions first gives any later assessment a clear starting point and may reveal an urgent filtration or temperature fault.

Make weekly maintenance a large-fish routine

OATA recommends, at least once every week, a partial water change of 25% for very large fish, together with water testing at least weekly. Treat that as the starting schedule. If nitrate rises too quickly, food remains uneaten or solid waste accumulates between changes, increase the effectiveness or frequency of maintenance rather than waiting for the water to look dirty.

Follow one repeatable service sequence

1. Test ammonia, nitrite, nitrate, pH and temperature before disturbing the aquarium.
2. Observe swimming, breathing and appetite so unusual behaviour is not hidden by maintenance activity.
3. Remove 25% of the water while siphoning accessible solid waste.
4. Treat replacement tap water with conditioner to remove chlorine or chloramine before it enters the aquarium.
5. Check the filter for blockages and confirm normal flow after the service.
6. Record the results so a gradual change is visible from one week to the next.

If filter media needs cleaning, rinse it lightly in aquarium water removed during the water change. Do not rinse biological media under the tap, because chlorine or chloramine may kill part of the beneficial bacterial population. Avoid cleaning every biological stage at once; preserving active media helps the filter continue processing the fish’s waste.

A large aquarium can look clear while dissolved waste is moving in the wrong direction. Weekly figures show whether feeding, filtration and water changes remain balanced as the pacu grows. Increase the maintenance capacity before the trend reaches the operating limit, rather than allowing the animal’s present size to outrun the routine.

Feed a plant-led diet with controlled portions

Wild Red-Bellied Pacu feed primarily on fruits, nuts and seeds, while also taking insects, zooplankton and small fish opportunistically. Translate that pattern into a varied, plant-led aquarium diet built around a quality dried pellet or floating stick suitable for a large herbivorous or omnivorous freshwater fish.

Build each week around a dependable staple

Offer the formulated food as the nutritional foundation, then rotate suitable plant foods. Seriously Fish lists spinach, lettuce, apple, banana, peach, grapes, courgette, peas, cabbage and carrot. Wash fresh produce, prepare pieces that match the fish’s current mouth size and introduce unfamiliar items gradually so acceptance and waste can be observed.

Feed once a day and offer only what the fish can eat within a few minutes. Watch the entire meal rather than relying on the pacu’s willingness to continue feeding. Remove uneaten food before it breaks down, because surplus food adds to the organic load and can push ammonia, nitrite or nitrate in the wrong direction.

Adjust the portion from what you observe

• If food remains after a few minutes, remove it and reduce the next portion.
• If the fish rejects a newly introduced item, return to accepted foods and try a smaller amount another day.
• If appetite drops suddenly, test water and inspect temperature and filter operation before changing the diet.
• If nitrate rises between weekly services, review both the quantity fed and how effectively fragments are removed.
• If growth increases the size of each meal, confirm that filtration and water-change capacity are keeping pace.

Appropriately sized food matters throughout growth. A pellet designed for a much smaller fish may scatter and enter inaccessible areas, while oversized pieces may be left partially eaten. Choose a form the pacu can consume cleanly and reassess that choice as its mouth and body size increase.

Keep the swimming route open and equipment protected

Décor is optional for an adult Red-Bellied Pacu, but open swimming space is essential. The species is active, so a complicated aquascape takes away useful room and creates more places for waste to collect. Begin with the swimming route, filtration access and safe equipment placement; add only furnishings that leave those functions intact.

Choose a simple, secure layout

Water-worn boulders, large pieces of driftwood or substantial beech branches can be used sparingly. Every item should be too heavy for the fish to move or secured so it cannot shift. Keep the central route broad and ensure that décor does not reduce the required front-to-back turning space.

Live plants are unlikely to remain a reliable feature because the fish may eat them immediately. A planted display should therefore not be part of the welfare plan or the expected long-term appearance. An uncluttered enclosure is easier to inspect, siphon and adapt as the animal grows.

A large specimen may damage glass heaters, thermometers or similar equipment as it moves around. Position vulnerable components outside the main enclosure where the filtration design permits, or protect them without creating a narrow trap. Equipment protection must still allow inspection and cleaning, because an inaccessible heater or blocked inlet cannot be maintained reliably.

Review the layout whenever the fish’s turning path changes. What leaves generous clearance for a small juvenile may become restrictive later. Removing a decorative item before it interferes with movement is better than retaining an attractive layout that no longer suits the animal’s body depth or activity.

Track growth, condition and system capacity together

Judge the aquarium as a changing system rather than a one-time purchase. As the pacu grows, food intake, solid waste and the space occupied by each turn all increase. Record measured length periodically, note body condition and compare those observations with water-test trends and maintenance time.

Use the first month to confirm the routine

• Week one: establish normal swimming, breathing, colour and appetite while checking ammonia and nitrite closely.
• Week two: confirm that once-daily feeding is consumed within a few minutes and leaves no persistent debris.
• Week three: compare nitrate before and after the weekly water change to see whether the routine is controlling dissolved waste.
• Week four: review swimming clearance, filter flow and the next growth-stage enclosure rather than waiting for visible crowding.

Healthy fish generally show clear, bright eyes, intact scales, undamaged fins, appropriate swimming and steady feeding. Look for changes from the individual’s established pattern: hanging at the surface, sitting on the bottom, erratic swimming, unusual hiding, a darker or paler appearance, altered breathing, white spots, fluffy growths, lost scales, damaged fins or declining body condition all warrant prompt attention.

When a change appears, test the water first and inspect temperature and filter flow. Record the readings and the time the behaviour began. If the signs persist, worsen or include visible injury or growths, seek advice from our livestock team or an aquatic veterinary professional rather than choosing a treatment without a diagnosis.

Plan the next capacity increase early

A temporary grow-on aquarium remains suitable only while it gives the juvenile adequate movement and the filter controls the feeding load. Have the next system mature before either limit is approached. At adult scale, use at least the 300 × 90 × 90 cm and 2,550-litre lifelong requirement, preferably larger.

Very large fish represent a substantial long-term commitment. Keep maintenance equipment, conditioned-water capacity and the filtration routine proportionate to the growing animal. A plan based on regular measurements is more dependable than assuming growth will slow to suit the enclosure.

Choose a social plan that still works at adult size

Red-Bellied Pacu show different social tendencies as they mature. Juveniles are shoaling fish, while adults become more solitary. Aquarium groups are possible only when the enclosure offers enough space, so juvenile grouping behaviour must not be used to justify a group whose adult requirements cannot be met.

Use one specimen as the clearest private plan

A single-specimen system removes the multiplying space and waste demands created by several adult pacu. It also makes feeding response, body condition and water-load changes easier to attribute to one animal. This is a planning recommendation rather than a claim that the fish must always be alone.

If a group is proposed, assess the complete adult group before choosing the juveniles. The system must allow each large, active fish to move without persistent chasing or fin nipping and must process the combined feeding load. Seriously Fish warns that adults in smaller quarters may chase and nip both their own kind and other species.

Watch social behaviour alongside water quality. Repeated chasing, damaged fins or a fish being displaced from food shows that the group plan needs intervention even when the chemical readings are good. Conversely, peaceful juvenile behaviour does not guarantee that limited quarters will remain suitable as the fish become larger and more solitary.

Every extra pacu adds another animal capable of reaching 88 cm. Calculate the enclosure, filtration and feeding commitment from that mature scale. Choose a group only when the complete lifelong arrangement is more than a temporary juvenile display.

Screen every companion by size and activity

Avoid much smaller fish because they are likely to be eaten. OATA gives the same precaution for very large fish generally, noting that their size and large mouths make mixing them with much smaller animals unsuitable. This rule applies even when a juvenile pacu appears peaceful in a dealer or grow-on aquarium.

Larger companions require a case-by-case assessment. Red-Bellied Pacu are generally peaceful with bigger fish, but their constant activity may bother sedentary or shy animals. A companion must be robust enough for the shared activity while remaining suitable for the same temperature, water chemistry and exceptionally large enclosure.

Apply the screen in this order

Similarly sized characins, large cichlids and large catfish are cited as possible companions where truly enormous space exists, but these group names are not automatic approvals. Individual behaviour, adult dimensions and feeding activity still decide whether a proposed combination is responsible. Request a stocking assessment from our team before committing to a mixed giant-fish system.

The tetras collection is useful when comparing smaller aquarium choices, not as a tank-mate list for this pacu. Small tetras fall directly under the avoid-much-smaller-fish rule.

When uncertainty remains, choose the single-specimen plan. It avoids exposing a smaller fish to predation and avoids placing a shy or sedentary animal beside the pacu’s constant movement. Compatibility should be reassessed as every occupant grows, because a pairing that appears balanced at dispatch size may become mismatched at maturity.

Answer the key Red-Bellied Pacu questions

What is a Red-Bellied Pacu?

Red-Bellied Pacu is a common name for Piaractus brachypomus, a giant South American freshwater fish in Serrasalmidae. Pirapitinga and Freshwater Pompano are other names for the same animal, and FishBase records a maximum of 88 cm total length.

What does one quantity of Red-Bellied Pacu include?

One quantity supplies one animal of the selected juvenile size. It is not a pair or group, and the live selector should be used to choose the required size option.

Which juvenile size options are used for Piaractus brachypomus?

The dispatch sizes are 2–3 cm, 3–4 cm, 5–6 cm, 7–8 cm and 10–15 cm. Each grade describes the size supplied rather than the potential 88 cm adult length.

What lifelong setup does Red-Bellied Pacu need?

Provide at least a 300 × 90 × 90 cm aquarium holding 2,550 litres for one adult, preferably larger. Keep a long open swimming route and use large, efficient biological filtration capable of processing the waste from a very large fish.

Should Red-Bellied Pacu be kept alone or in a group?

Juveniles are shoaling, while adults become more solitary. A one-fish plan is clearest for a private system; keep a group only when an exceptionally large aquarium can support every animal at adult size without crowding, chasing or fin nipping.

What water suits this fish?

Keep it at 23–28°C, pH 4.8–7.5 and 0–15 dGH, avoiding abrupt changes and extremes. Maintain ammonia and nitrite at zero mg/L and nitrate no more than 20 mg/L above the normal tap-water level.

What should I feed it?

Feed quality dried pellets or floating sticks as the staple and add varied leafy greens, vegetables and prepared fruit. Offer one meal a day, limit it to what is eaten within a few minutes and remove leftovers before they decay.

How should I acclimate this fish?

Prepare a mature aquarium within 23–28°C and confirm zero mg/L ammonia and nitrite before arrival. Avoid abrupt temperature or chemistry changes and observe breathing and balance during the settling period.

Can it live with other fish?

Avoid much smaller fish because they are likely to be eaten. Larger companions need an individual assessment covering adult size, activity, feeding, water requirements and the combined waste load in a truly enormous aquarium.

Check the care references

• FishBase: Piaractus brachypomus — accepted identity, family, distribution, environmental values and maximum total length.
• Seriously Fish: Piaractus brachypomus — lifelong aquarium dimensions, aquarium water range, feeding, furnishings and compatibility.
• OATA: How to look after very large fish — UK group guidance on water quality, introduction, feeding, maintenance and health observations.
• RSPCA aquarium environment guidance — biological filtration and the processing of ammonia, nitrite and nitrate.