The Harlequin Rasbora Trigonostigma heteromorpha (previously Rasbora heteromorpha) is immensely popular. Theseextremely desirable fishare always on the move. A lively school at the top of the aquarium hasan endearing, fiery persona that just radiates attitude. This cyprinid has been a favorite aquarium fish for about 100 years. It was first described by Duncker back in 1904 and hails from Southeast Asia. It is found on the Malay Peninsula, throughout western Malaysia and southern Thailand as well as Singapore.

These small, elegantfish look their best when kept In ideal conditions. They won’t quite reach 2 inches (5 cm) in length, and they can be variable in color, rangingfrom a pale pink or bright red to a copper orange along the top and bottom. The back halves of these little fish havea black triangular or hatchet-shaped patch with a bluish tint. They will sometimes have subtle purple highlights in front of the patch as well.

With these decorative colors, they are aptly dubbed Harlequin Rasbora, or sometimes just Harlequin or Harlequin Fish. Most individuals havea rosyhue,so another common name for them is Red Rasbora. However, varieties that are more copper or golden are sometimes called Gold Harlequin Rasbora. Varieties with a black patch and a mostly black body are called Black Harlequin Rasbora, and those with a strong bluish tinting will sometimes be referred to as Blue Harlequin Rasbora.

This fish is often confused with two of its close relatives, the Glowlight RasboraTrigonostigma hengeli and the Lambchop Rasbora Trigonostigma espei. To accurately identifythem, pay attention tosome subtle distinctions. The Harlequin Rasbora is pale pink to bright redand much stockier than its cousins. Harlequin’s black mark is moretriangularand will have a noticeably blue tint. The Espei Rasbora should be bronze pink in color andmore elongated. Hengel’s Rasbora should be yellow cream with a thin black side marking topped with a sweep of neon orange.

These agreeable fishdo fine in most community settings and even better with other cyprinids. They aren’t aggressive per se but can be a bit boisterous. The companionship of their own species is absolutely essential, so a school is a must. You really haven’t seen the Harlequin Rasbora until you’ve seen a school of 10 or moreindividuals in an established tank. Because theseareexcellent schooling fish, they need a fairlylargeaquarium. A 20-gallon tank would be the bare minimum, but they are bestsuited to a 30-gallon aquarium that is at least 36 inches long toallow them to swim freely.

They are very tolerant fish but do require a little extra care, so they are suggested for aquarists with a bit of experience. They can adapt to a variety of conditionsbut need soft, slightly acidic water. Theywon’t do well when kept in hard water. To bring out their best colors, use a darksubstrate and provide them with plenty of plants. A well-planted aquarium will really showcase their colors to best effect. Keep the tankcarefully covered as these fish are liable to jump if startled or excited.

Harlequin Rasboras

Scientific Classification


Harlequin Rasbora – Quick Aquarium Care

Aquarist Experience Level:Intermediate
Aquarium Hardiness:Very Hardy
Minimum Tank Size:20 gal (76 L)
Size of fish – inches2.0 inches (5.00 cm)
Temperature:73.0 to 82.0° F (22.8 to 27.8&deg C)

Habitat: Distribution / Background

The Harlequin Rasbora Trigonostigma heteromorpha (previously Rasbora heteromorpha) was described by Duncker in 1904. It is found in Southeast Asia on the Malay Peninsula throughout western Malaysia and southern Thailand as well as Singapore. It wasalso beenrecorded inIndonesia (Sumatra). It is known to occur on the island of Bintan in Riau Islands province, Sumatra, but it is not believed to occur on the mainland. Early records are now thought to refer to another species.

There are currently four distinct members in the Trigonostigma genus. These fish were previously contained in the Rasbora genus but were reclassified in 1999 by Kottelat and Witte. They were distinguished by their triangularpatterning and their coloring, as well as their reproductive method of depositing adhesive eggs underneath leaves or other matter rather than simply scattering them.

This species is listed on the IUCN Red List as Least Concern (LC). There has been some impact on populations in Thailand, but in Malaysia, it is stable with no major concerns. Other common names it is known by include Red Rasbora, Harlequin Fish, and Harlequin. Color variations are sometimes called Black Harlequin Rasbora, Blue Harlequin Rasbora, and Gold Harlequin Rasbora.

In nature, it inhabits forest streams and is primarily found in peat swamps withheavily vegetatedwaters that are calm or gently flowing. Due to the presence ofdecomposing organic debris, the water is sometimes stained a yellowish brown by tannins and other chemicals. The wateris soft and weakly acidic or neutral, and the forest canopies often keep it shaded. These fish swim in schools andare known as micro-predators, feeding on small insects, worms, crustaceans, and zooplankton.

  • Scientific Name: Trigonostigma heteromorpha
  • Social Grouping: Groups
  • IUCN Red List: LC – Least Concern


The Harlequin Rasbora is a small fish, reaching a total length of 2 inches (5 cm). It is much stockier than its cousins and is rather ‘tall’ with a convex belly area.

Depending on the region of origin, these fish will vary in appearance. The fish in Thailand are normally smaller and slimmer than the fish from Singapore and the Sunda Island populations. It is thought that these deviations are caused by captive-bred fish being released or escaping from fish farms.The average lifespan is about 5 to6 years in the aquarium with good care.

These fishcanvaryin color,rangingfrom a pale pink or bright red to a copper orange along the top and bottom. Ablack triangularmarking with a noticeably blue tint adornsthe back half of its side, parallel to the gills. Some individuals will havepurple highlights just in front of the patch.

Varieties witha morecopper or golden color are sometimes called Gold Harlequin Rasbora whilethosewith a black patch and a mostly black body are called Black Harlequin Rasbora. Those with a strong bluish tinting will sometimes be referred to as Blue Harlequin Rasbora.

  • Size of fish – inches: 2.0 inches (5.00 cm)
  • Lifespan: 5 years – They have an average lifespan of 5 to 6 years with proper care.

Fish Keeping Difficulty

The Harlequin Rasbora is suggested for an aquarist with some fish keeping experience. These fish are fairly hardy and will adapt to most aquarium conditionsbut optimally should have soft, slightly acidic water. They won’t do well if kept in hard water, particularly if you want to breed them. They are usually not very picky eaters andwill accept and thrive on quality flake foods. With well filtered water and regular maintenance, these fish will do very well, but they must be kept in a good-sized school of 10 or more.

  • Aquarium Hardiness: Very Hardy
  • Aquarist Experience Level: Intermediate

Foods and Feeding

Harlequin Rasboras areomnivores, and in the wild, they feed on small insects, worms, crustaceans, and zooplankton. They should be given a balance ofquality flake or pellet food supplemented with the occasional live snack ofbrine shrimp or bloodworms. Although live foods are an important part of their diet, don’t over indulge them, or they may refuse prepared foods.

Some blanched lettuce or spinach is also a welcome addition to their diet. These fish will do best when offered food several times a day, but only offer what they can eat in 3 minutes or less at each feeding. If you feed only once per day, provide what they can eat in about 5 minutes.

  • Diet Type: Omnivore
  • Flake Food: Yes
  • Tablet / Pellet: Yes
  • Live foods (fishes, shrimps, worms): Some of Diet
  • Vegetable Food: Some of Diet
  • Meaty Food: Some of Diet
  • Feeding Frequency: Several feedings per day – Offer only what they can consume in 3 minutes or less with multiple feedings per day.

Aquarium Care

They are not exceptionally difficult to care for provided their water is kept clean. At least 25 to50% of the tank water should be replaced once a month. If the tank is densely stocked, 20 to25% should be replaced weekly or every other week. The substrate should also be vacuumed during water changes to avoid accumulation of waste.

  • Water Changes: Monthly – If the tank is densely stocked, water changes should be done every other week.

Aquarium Setup

Harlequin Rasboras are a fairly hardy schooling species andwill spend much of their time in the open water ofthe middle and upper regions of the aquarium. A school of 10 or more will need plenty of room to swim around and a tight cover to prevent jumps. A 20-gallon tank isthe smallest needed to house a school,but they will do best in a spacious aquarium of 30 or more gallons. Moreimportantly, they should be keptin a long aquarium of 36 or more inchesto allow them to swim freely. Although they can adapt to most aquarium conditions, their colors can be quite stunning and will show best in soft, slightly acidic water conditions. Provide a good filter, but as these fish come from very sluggish waters, strong filtration is not necessary.

A heavily planted tank will make them feel much more at home. A good aquascape would be dense plantings reaching the surface of the aquarium around the sides and back and a few broad-leaved plants for shelter. A dark substrate and areas of shadow will bring outtheir best colors. Usinghardy floating plants to create shadows will give thisfish extra security and help to diffuse the light entering the tank. Bogwood is also a great addition, as the tannins help maintain water parameters more closely resemblingtheir natural environment. To create a natural feel and encourage the growth of microbe colonies, add dried leaves to the tank. The microbe colonies will also act as a secondary food source.

  • Minimum Tank Size: 20 gal (76 L) – These fish do best in large groups, so a bigger than normal tank is needed.
  • Suitable for Nano Tank: Yes
  • Substrate Type: Any – Adding some dried leaves to the substrate will help simulate their natural environment.
  • Lighting Needs: Moderate – normal lighting
  • Temperature: 73.0 to 82.0° F (22.8 to 27.8&deg C)
  • Breeding Temperature: 77.0° F – Breeding temperatures are between 77 and 82.4° F (25 – 28° C)
  • Range ph: 5.5-8.0
  • Hardness Range: 1 – 15 dGH
  • Brackish: No
  • Water Movement: Moderate
  • Water Region: All – They will spend much of their time swimming through open water in the middle or upper regions.

Social Behaviors

They are generally good community fish but are quite active and have big personalities. They must be kept in a group of 10 or more individuals for their well being. The males will display their best colors as they compete for the attention of females. These fish may chase each other about a bit, but without any real consequence.

They will get along with most fish, but someless active tankmates can’t tolerate the fast-moving lifestyle of the rasboras. Additionally, they are small and easily eaten by bigger tankmates. Good tankmates include many popular community fish. Cyprinids are especially good as well as some of the characins,like tetras, and live bearers like mollies, guppies, plates, and swordtail fish. They also do well with bottom-dwelling peaceful catfish and loaches, and even some of the dwarf cichlids. However, they will suffer in the company of overly large and aggressive fish.

  • Temperament: Peaceful – They are peaceful fish but will get nervous with larger tankmates.
  • Compatible with:
    • Same species – conspecifics: Yes – This fish must be kept with a school of at least 10 of its own kind, and more is better.
    • Peaceful fish (): Safe
    • Semi-Aggressive (): Threat – Should be kept with peaceful community fish only.
    • Aggressive (): Threat
    • Large Semi-Aggressive (): Threat
    • Large Aggressive, Predatory (): Threat
    • Safe
    • Shrimps, Crabs, Snails: Safe – not aggressive
    • Plants: Safe

Sexual differences

Females are slightly larger and plumper than males. On the males, the black triangle is slightly more rounded and elongated.

Breeding / Reproduction

Breeding the HarlequinRasbora can be a bit challenging. Water conditions must be optimal for them to be willing to spawn, and then they can spawn for days before an actual mating takes place. Like other cyprinids, they exhibit no parental care for the young, but they differ slightly in their spawning method. Rather than being open water egg scatterers, they attach theireggsto the undersides of broad-leaved plants or other objects. Pairs need to be well conditioned with small offerings of live foods several times a day, usually for about 4 weeks. Conditioning with live foods will also bring out the males’ color. When well fed, mature females should begin filling out with eggs.

It is best to provide a dimly lit breeding tank and keep the water level low, at about 6 to8″ (15 – 20 cm). The water should be very soft and not higher than 1.5 to2.5° dGH, slightly acidic with a pH of 5.3 – 6.0, and a temperature between 77 and82.4° F (25 – 28° C). Filtration isn’t really necessary, but you can add a small air-powered sponge filter or some peat filtration. They need either artificial or live broad-leaved plants, like Microsorium or Cryptocoryne, for the eggs to adhere to. A spawning mop can also be added, so any eggs that don’t attach to the leaves have a place to fall that’s out of the reach of the parents.

Afterthe pair are moved tothe breeding tank,spawning will usually occur in the morning hours. The male will direct the female to his selected location, almost always the underside of a broad-leaved plant. The male performs a courtship dance, and the spawn takes place under the plant leaves, with the partners swimming upside down. The pairwill spawn, and 6 to12 eggs will be deposited at a time on the underside of a leaf. About 80 to100 eggs are typical, though they could deposit as many as 300.

Although they take much care in depositing the eggs, they will eat their eggs and fry if not removed. After the spawn, remove the parents and darken the tank. At this time, you can lower the water level to between 4 and6 inches (10-15 cm), taking care to keepthe eggsbelow the surface.

The fry will hatch in about 24 hours (usually between 18 and36) and be free-swimming in about 3 to5 days. The free-swimming fry are only 3 to4 mm long so provide starter foods like infuser for the first few days. They grow quickly and will be large enough to eat baby brine shrimp after about a week to 14 days. See Fish Food for Fry for information about types of foods for raising the young.

  • Ease of Breeding: Moderate

Fish Diseases

Harlequin Rasboras are very hardy, and disease is not usually a problem in a well-maintained aquarium.
Some diseases thatare common in rasboras are dropsy, fin rot, and Ich if good water quality, nutrition, and maintenance is not provided. With any additions to a tank, such as new fish, plants, substrates, and decorations, there is a risk of introducing disease. Properly clean or quarantine anything you want add to an established tankso as not to upset the balance.

These fish are very resilient, but knowing the signs of illness, and catching and treating them early makes a huge difference. An outbreak of disease can often be limited to just one or a few fishes if dealt withat an early stage. The best way to proactively prevent disease is to give your fish the proper environment and a well-balanced diet. The more closely their environment resemblestheir natural habitat, the less stress the fish will have, making them healthier and happier. A stressed fishis more likely to acquire disease. For information about freshwater fish diseases and illnesses, see Aquarium Fish Diseases and Treatments.


The immensely popular Harlequin Rasbora is available basically anywhere, both in stores and online, and is generally inexpensive. They occur in a variety of color patternsand are sometimes called the following:

  • Black Harlequin Rasbora – strong black patch and a mostly black body
  • Gold Harlequin Rasbora – copper or golden variety
  • Blue Harlequin Rasbora – strong bluish tinting.



Featured Image Credit: Joan Carles Juarez, Shutterstock