Harlequin (Rasbora heteromorpha)

Choose a peaceful Harlequin shoal

Harlequin (Trigonostigma heteromorpha) is a small freshwater fish suited to a calm, well-maintained community aquarium. Its social behaviour, peaceful temperament and manageable adult dimensions make it a practical choice for a keeper who can provide a proper group, stable water and uninterrupted swimming space. This is also a suitable first community species for a newcomer prepared to mature the aquarium before introducing fish and maintain it every week.

FishBase lists Trigonostigma heteromorpha as the accepted name and treats Rasbora heteromorpha as a synonym. It places the species in Danionidae and records a maximum length of 5 cm total length for male or unsexed fish. Use that 5 cm maximum when planning adult stocking: the selected dispatch grade describes the animal sent, not the space it will eventually require.

The natural distribution recorded by FishBase covers the Malay Peninsula, including Peninsular Malaysia and Singapore, and Indonesia. It is a tropical, benthopelagic freshwater fish. Its recorded environmental values are 22–25°C, pH 5.0–7.0 and 5–12 dH; aquarium husbandry guidance is broader and is set out in the care sections below.

Decide whether it fits your aquarium

Seriously Fish describes the Harlequin as very peaceful and recommends it for newcomers to fishkeeping. That suitability depends on providing the group, footprint, biological filtration and regular maintenance described here; it is not a reason to place the fish in a new or undersized aquarium.

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.

Reference: reviewed public source.

Select the dispatch size and quantity

One purchase quantity supplies one animal of the selected option. The size you select is the size supplied, not the adult size. Use the live size selector to see the options that can be chosen when ordering, then set the purchase quantity to the number of animals required for your planned group.

The size selector grades are 1.2–1.5 cm, 1.5–1.9 cm, 2 cm, 2.5–3 cm, over 3 cm and XXL. XXL is a named grade rather than a numeric adult-size promise. Every grade represents a dispatch size, while the aquarium must be planned around the recorded maximum of 5 cm total length.

Translate the selector into a group

• One selected animal: choose a size grade and set the quantity to one.
• A group of five: choose the required grade and set the quantity to five, because each purchase unit supplies one fish.
• An existing shoal: count the Harlequins already present, assess the aquarium’s remaining capacity and add only the number the mature filter and footprint can support.
• A mixed community: reserve space for each species at its adult size before deciding the Harlequin quantity.

Do not use a small dispatch grade to justify a smaller aquarium or heavier stocking. A 1.2–1.5 cm animal still needs an adult plan based on a 5 cm fish, and several small arrivals still increase the waste load handled by the biological filter. Add livestock gradually so the beneficial bacterial population can adjust as feeding and waste increase.

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

Before ordering, confirm the aquarium has a base of at least 60 × 30 cm or an equivalent footprint, a volume of about 54 litres or more, a mature filter and water within 21–28°C, pH 5.0–7.5 and 1–12 dGH. The tank also needs room for a group and any existing residents once every fish reaches its adult size.

Browse the rasboras and danios collection when comparing social freshwater fish, but assess each animal by its own adult size, water range and temperament. For dispatch days, packing, claims and the Live Arrival Guarantee, read our delivery and live-arrival policy before completing the order.

Check whether this animal suits the aquarium

Plan the aquarium around the adult size

Start with a base measuring at least 60 × 30 cm or an equivalent footprint and a volume of approximately 54 litres. Seriously Fish describes that as the smallest aquarium to consider because this fish should be maintained in numbers. A longer or larger aquarium gives the group more usable swimming room and leaves more capacity for carefully screened community companions.

OATA’s rasbora sheet gives a group of at least five as broad guidance for rasboras, with more being better; use five as the minimum planning point for Harlequins and increase the group only when the aquarium’s footprint, filter capacity and existing population allow it. Keeping too few rasboras may cause stress, so a single specimen is not an appropriate long-term plan.

Work from the aquarium backwards

1. Measure the usable base, not just the advertised litre capacity. The starting footprint is 60 × 30 cm.
2. List every existing fish and its adult dimensions rather than its present size.
3. Reserve room for at least five Harlequins at the recorded 5 cm maximum.
4. Consider swimming space, shelter and the activity of other residents, not litres alone.
5. Check that the established filter can accommodate the planned increase in feeding and waste.

A 54-litre aquarium with the required base is a starting point for the Harlequin group, not an automatic allowance for additional species. An aquarium with the same volume but an unusually narrow or short base may offer less useful horizontal space. Conversely, a larger footprint can make it easier to separate planted shelter from open swimming areas, but it does not remove the need to control stocking and water quality.

For an empty aquarium, plan the full adult community before purchasing the first animal. For an established aquarium, treat any new group as an increase in biological load. Add fish slowly and continue testing after each addition rather than filling every planned place at once.

Hold stable water within the aquarium range

Maintain the aquarium at 21–28°C, pH 5.0–7.5 and 1–12 dGH. These are the species aquarium ranges given by Seriously Fish. They describe workable limits rather than a reason to make frequent adjustments, so choose a stable point that also suits every other resident and avoid repeatedly moving the values around.

Test the aquarium rather than relying on assumptions about local tap water. Our water chemistry guide explains the values to measure, while the water matcher helps compare measured results with the Harlequin ranges. Record the readings so gradual movement can be recognised before it becomes a sudden problem.

Use each value for a clear decision

Temperature, pH and hardness must work together. A prospective tank mate that fits the temperature range but requires water outside pH 5.0–7.5 or 1–12 dGH is not a sound match. The same rule applies in reverse: shared chemistry does not make an aggressive, boisterous or predatory animal suitable.

Use treated replacement water during maintenance. Tap-water conditioner is required where chlorine or chloramine is present, because either can damage fish and the beneficial bacteria in the filter. Prepare the replacement water before it enters the aquarium and avoid a maintenance routine that causes abrupt changes in temperature or chemistry.

If a reading moves outside the working range, first check the test method and compare the aquarium with its usual result. Then look for a blocked filter, excess food, accumulated waste or an overly rapid increase in livestock. Correct the cause steadily; chasing individual readings with repeated large interventions can create further instability.

Match filtration and heating to the aquarium

Use a mature biological filter in every Harlequin aquarium. The RSPCA explains that biological filtration supports beneficial bacteria that convert toxic ammonia to toxic nitrite and then to the safer nitrate. That process protects the fish only when the filter is established, maintained and sized for the aquarium’s livestock load.

For an aquarium of about 54 litres, Seriously Fish gives a turnover of four to five times the volume per hour. That corresponds to a filter flow rating of approximately 215–270 litres per hour. Treat the rating as a selection guide: media condition, blockages and the aquarium layout can affect the actual circulation.

Choose equipment from the working numbers

A filter rating is not permission to overstock. Biological capacity depends on a living bacterial population, and that population needs time to grow when more fish and food are added. Introduce livestock gradually and monitor ammonia and nitrite rather than assuming that a powerful pump has made the aquarium safe.

Prepare and mature the system before the group arrives. Our first tropical aquarium guide covers the setup sequence. The decisive checkpoints are a working filter, stable temperature and test results showing 0 mg/l ammonia and 0 mg/l nitrite.

Check the filter for blockages during weekly maintenance. If its media needs cleaning, do not run it under the tap. Rinse it lightly in water removed from the aquarium. Chlorine or chloramine may kill the bacteria the media is meant to support.

Watch the fish and the water after any equipment change. Hanging at the surface, unusual bottom-sitting, erratic swimming or altered breathing requires prompt water testing. Also confirm that the filter is running, the heater is maintaining the selected temperature and circulation has not been restricted by debris.

Build cover around open swimming space

Create a well-furnished aquarium while preserving clear swimming room for the group. Seriously Fish notes that décor is not especially critical, but Harlequins tend to show better colour in a furnished setup with a dark substrate. OATA also recommends plant cover and open swimming space for rasboras, so arrange shelter around usable water rather than filling the whole base with ornaments.

Use a simple layout

• Substrate: choose soft sand for a natural-looking arrangement or suitable aquarium gravel.
• Structure: place wood roots and branches so they create shade and sheltered spaces.
• Planting: use live plants where possible, particularly around the back and sides, while retaining open water.
• Alternative cover: if live plants are not used, provide aquarium-safe artificial plants and ornaments that create shelter.
• Flow: arrange décor so it does not obstruct the filter or leave debris trapped in inaccessible areas.

Dried leaf litter can add another layer of cover and help create a blackwater-style display. Seriously Fish names beech, oak and Ketapang almond leaves as suitable choices. As leaves decompose, beneficial microbial colonies can develop and provide a secondary food source for fry. Tannins and other compounds released during decomposition also help simulate blackwater conditions.

Leaves may be left to break down fully or removed and replaced every few weeks. Whichever approach you choose, inspect them during maintenance and siphon unwanted accumulated waste. Add leaf litter deliberately rather than allowing it to smother the substrate, block a filter intake or occupy the group’s main swimming area.

A furnished aquarium supports confidence, but décor cannot compensate for an undersized footprint, unsuitable water or too small a group. Make the footprint and stocking decisions first, then use substrate, plants, wood and leaf litter to create a practical balance between shelter and swimming room.

Feed small foods once each day

Feed a varied diet built around good-quality flakes or granules, supplemented with small live or frozen foods. Wild stomach analyses reported by Seriously Fish show that Harlequins act as micropredators, feeding on small insects, worms, crustaceans and other zooplankton. In an aquarium, regular variety supports condition and colour.

Suitable live or frozen foods include bloodworm, Daphnia and Artemia. Use pieces that the fish can take readily and distribute the portion so the group has a fair chance to feed. A good-quality dried food remains the practical staple, while the live or frozen items add variety.

Use one controlled daily feed

Watch the whole group during the meal. Food should be eaten within a few minutes rather than sinking into décor or remaining on the substrate. If food is left, reduce the next portion and remove accessible waste during maintenance. Uneaten food breaks down and adds waste to the water, increasing the pressure on the biological filter.

Use feeding response as a daily observation point. Reduced intake or a loss of interest in food can accompany stress or declining health. If one animal is consistently missing food, check whether faster residents are taking the whole portion, whether the food is an appropriate size and whether the group is showing any wider change in swimming, breathing, colour or condition.

Do not respond to poor appetite by repeatedly adding different foods in large amounts. Test ammonia and nitrite, confirm the temperature is stable and observe whether other fish are affected. A water-quality or equipment problem needs correction at its cause; extra food can make the waste load worse.

Variety means rotating suitable foods, not serving several full portions at once. A simple pattern is a quality dried staple on most days with an appropriately small live or frozen item used regularly in place of, or as part of, the controlled daily allowance.

Introduce the group to a mature aquarium

Introduce Harlequins only after the aquarium is mature, filtered and stable. Confirm ammonia and nitrite are both 0 mg/l, the temperature is holding within 21–28°C, and pH and hardness fit the species range. Make these checks before the fish are added rather than relying on the aquarium’s age or appearance.

Stock the aquarium gradually. Every increase in fish and feeding increases the demand placed on the filter’s beneficial bacteria. Adding too much livestock too quickly can lead to new tank syndrome, in which the bacterial population cannot process the increased waste and ammonia or nitrite rises to harmful levels.

Use a staged introduction check

1. Before introduction: test temperature, pH, hardness, ammonia and nitrite; confirm the filter and heater are running correctly.
2. At introduction: follow the selected livestock introduction method carefully and avoid adding unrelated groups at the same time.
3. After introduction: check that the fish show appropriate swimming behaviour and inspect the equipment for interruptions.
4. During the first week: test at least once and test sooner if swimming, breathing, feeding or colour changes.
5. Before further additions: allow the aquarium to remain stable and reassess adult stocking, filter capacity and compatibility.

A normal introduction plan does not end when the fish enter the aquarium. Observe clear, bright eyes, intact scales, undamaged fins and appropriate swimming. Also look for surface hanging, bottom-sitting, erratic movement, unusually fast or slow gill-cover movement, increased hiding or reduced interest in food.

Do not add more fish merely because the first group appears settled. Stable behaviour must be supported by stable test results. Ammonia and nitrite should remain at 0 mg/l, and nitrate should be controlled through regular partial water changes.

If a reading changes after introduction, pause further stocking. Check for uneaten food, equipment failure, a blocked filter or an increase in waste. Correct the cause and wait for stable conditions before reconsidering the next addition.

For an existing community, complete the compatibility screen before introduction. The aquarium must have room for the Harlequin group at 5 cm each, calm companions, shared water requirements and an established biological filter able to process the extra load.

Complete the weekly maintenance routine

At least once every week, carry out a partial water change of 25%. Use a siphon to remove solid waste from the substrate and from accessible spaces around wood, plants and leaf litter. This routine removes accumulated waste while giving you a regular opportunity to inspect the aquarium and its equipment.

Follow the same sequence each week

1. Test ammonia, nitrite and nitrate before the water change so the readings describe the aquarium’s condition.
2. Inspect the fish for changes in swimming, breathing, colour, appetite, fins, scales and body condition.
3. Switch off equipment as required for safe maintenance.
4. Siphon 25% of the water while removing visible solid waste.
5. Treat replacement tap water with conditioner to remove chlorine or chloramine.
6. Check the filter for blockages and confirm normal operation after maintenance.
7. Record the results and compare them with previous weeks.

Keep ammonia at 0 mg/l and nitrite at 0 mg/l. OATA advises that nitrate should not exceed 20 mg/l above the normal tap-water level. Testing both the aquarium and source water helps distinguish nitrate entering with replacement water from nitrate accumulating through feeding and fish waste.

Clean filter media only when its condition or reduced flow requires attention. If the media needs cleaning, do not run it under the tap. Rinse it lightly in water removed during the partial change. This limits the loss of beneficial bacteria.

A larger water change is not a substitute for diagnosing a repeated problem. If ammonia, nitrite or nitrate control is poor, examine stocking, feeding, filter performance and trapped waste. Weekly records make patterns easier to recognise than isolated test results.

Finish by confirming the heater and filter have restarted and the temperature is returning to the chosen stable point. Observe the group after maintenance so equipment interruptions or unusual behaviour are noticed promptly.

Respond to changes before they escalate

Learn the group’s normal appearance, swimming, breathing and feeding response. Daily familiarity makes a change easier to recognise, while weekly testing helps connect behaviour with water quality and equipment performance.

Use visible changes as action signals

Start with measured conditions rather than treatment. Confirm ammonia and nitrite are 0 mg/l, check temperature, inspect the filter and review recent additions or changes. Correcting a water-quality or equipment problem is an immediate husbandry action and may prevent further deterioration.

Do not increase food when appetite falls unless observation shows that the normal portion is simply being taken by faster fish. Extra uneaten food increases waste and can worsen ammonia or nitrite problems.

Changes affecting several residents at the same time make an aquarium-wide issue more likely, so check water and equipment first. A problem affecting one fish still requires prompt attention, but the same water checks help exclude a shared environmental cause.

If a health change persists, worsens or involves visible lesions, abnormal growths, substantial weight change or difficult breathing, contact us for appropriate next steps. Avoid selecting medication from a symptom alone, especially in a mixed aquarium where other fish, plants or invertebrates may respond differently.

Screen every community companion

Harlequins are very peaceful fish suited to a well-maintained community aquarium. Seriously Fish lists other small cyprinids, tetras, livebearers, dwarf cichlids, catfishes and loaches among the broad types that may be considered. Those categories are starting points rather than automatic approvals: each proposed species still needs to pass the adult-size, temperament, water and space checks.

Apply four checks in order

1. Adult size: reject any plan in which another animal may treat a 5 cm Harlequin as prey.
2. Temperament: avoid boisterous companions that may prevent this calm group from feeding or using the aquarium normally.
3. Water: require an overlapping long-term range within 21–28°C, pH 5.0–7.5 and 1–12 dGH.
4. Space: fit the full Harlequin group and every other resident at adult size without removing the required swimming room or shelter.

Shared water values are necessary but not sufficient. A candidate can fit the temperature and pH yet fail because it is predatory, territorial, excessively active or too large for the footprint. Conversely, a peaceful small fish is still unsuitable if its water requirements sit outside the aquarium’s stable range.

Give compatibility decisions to the exact animals involved rather than relying only on a common category name. Temperament, adult dimensions and space use can differ among closely related fish. Ask us for a stocking and compatibility assessment when you can provide the aquarium dimensions, water readings, filter, existing livestock and proposed quantities.

Balance space, water and temperament

Build the community around the aquarium’s real limits. Start with the Harlequin group, reserve its 60 × 30 cm minimum footprint and about 54 litres, then add the adult requirements of every proposed companion. The minimum Harlequin setup is not spare capacity for an unrestricted mixed community.

Work through a proposed match

Suppose a candidate fish is peaceful and remains small. First compare its temperature, pH and hardness needs with 21–28°C, pH 5.0–7.5 and 1–12 dGH. If the ranges overlap, choose a stable point suitable for both. Next compare adult sizes and behaviour: the candidate must not be able to eat a 5 cm fish, monopolise food or dominate the open swimming area. Finally, calculate whether both social groups fit once fully grown.

A second candidate may share the same water range but be highly active or boisterous. That match fails the temperament screen even though the chemistry appears suitable. A third may be calm but require water outside the Harlequin range; that match fails the water screen. Passing one check never cancels a failure in another.

Introduce one planned addition at a time. This gives the biological filter time to adapt and makes behavioural changes easier to attribute. Pause if ammonia or nitrite becomes detectable, feeding access changes, the Harlequins hide more than usual or swimming becomes erratic.

Use plants, wood and shelter to reduce exposure, but do not use décor to force incompatible fish together. A layout can create resting areas and visual separation; it cannot make a predatory, boisterous or water-mismatched species safe.

Protect the group with daily observation

Compatibility remains an active decision after introduction. A combination that appears workable from adult size and water values must also allow every Harlequin to swim, feed and use shelter normally. Observe the whole group rather than judging the aquarium by the most confident animal.

Watch the moments that reveal pressure

• Feeding: confirm calm companions do not take the entire portion before the Harlequins can eat.
• Open-water use: notice whether the group has been displaced into corners or dense cover.
• Rest and shelter: look for persistent hiding rather than occasional use of cover.
• Breathing: respond to unusually fast or slow gill-cover movement with immediate water and equipment checks.
• Body condition: inspect fins, scales and weight for developing changes.

A small amount of movement between open water and cover is not enough to diagnose a compatibility problem. Look for a sustained change from the group’s usual behaviour, then combine observation with water tests. Detectable ammonia or nitrite can cause distress that resembles social pressure, while actual harassment can coexist with acceptable chemistry.

Do not solve crowding by reducing the Harlequins below the minimum group plan. If the aquarium cannot support the social needs of every resident, change the stocking plan instead. The long-term goal is a calm community in which the Harlequin group has adequate numbers, clear swimming space, stable water and reliable access to food.

Answer Harlequin buying questions

What is Harlequin (Trigonostigma heteromorpha)?

It is a peaceful freshwater shoaling fish in Danionidae. FishBase treats Rasbora heteromorpha as a synonym and records a maximum of 5 cm total length.

How many Harlequin fish does one purchase supply?

One purchase quantity supplies one animal of the selected option. Set the quantity to the number required for the planned group.

Which size options can I select for a Harlequin?

The dispatch sizes are 1.2–1.5 cm, 1.5–1.9 cm, 2 cm, 2.5–3 cm, over 3 cm and XXL. Check the live selector when ordering.

What aquarium setup does a Harlequin need?

Use at least about 54 litres with a base of 60 × 30 cm or an equivalent footprint, mature biological filtration, shelter and open swimming space.

Should a Harlequin be kept alone or in a group?

Keep it in a group rather than alone. OATA’s rasbora guidance gives at least five as the group minimum, with more considered only when space and filtration allow.

What water conditions does this fish need?

Maintain 21–28°C, pH 5.0–7.5 and 1–12 dGH, with ammonia and nitrite at 0 mg/l. Keep the chosen values stable.

What should it eat?

Feed a good-quality flake or granule once daily, using only what is eaten within a few minutes. Add small Daphnia, bloodworm or Artemia for variety.

How should it be introduced to the aquarium?

Before introduction, confirm the aquarium is mature, ammonia and nitrite are 0 mg/l, the temperature is stable within 21–28°C and the filter is operating correctly.

Which tank mates suit this fish?

Choose calm companions with overlapping water needs that cannot eat a 5 cm fish. Avoid boisterous tank mates and check total adult stocking before mixing species.

Consult the care references

• FishBase: Trigonostigma heteromorpha — accepted identity, synonym, family, distribution, environmental values and maximum recorded length.
• Seriously Fish: Trigonostigma heteromorpha — aquarium footprint, volume, water range, filtration, décor, diet, behaviour and compatibility.
• OATA: How to look after rasboras — group care, water-quality limits, feeding, introduction, maintenance and health observations.
• RSPCA aquarium environment guidance — biological filtration and the processing of ammonia, nitrite and nitrate.
• Tropical Fish Co delivery and Live Arrival Guarantee policy — dispatch schedule, customer-service routes and claims process.