Harlequin Rasbora

Red Rasbora, Harlequin Fish, Harlequin

Family: Cyprinidae Harlequin Rasbora, Trigonostigma heteromorph, Red RasboraTrigonostigma heteromorphaPhoto © Animal-World: Courtesy Courtesy Barbara Roth
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Ok two mollies The hatchets were not purchased  Andrew

Harlequin Rasboras are ideal for people usually bored with fish, rivaling the flashy Neon Tetras in popularity!

The Harlequin Rasbora Trigonostigma heteromorpha (previously Rasbora heteromorpha) is immensely popular. These are extremely desirable fish as they are always on the move. A lively school at the top of the aquarium creates an 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 are small fish, elegant looking fish when kept In ideal conditions. They won't quite reach 2 inches (5 cm) in length and they can be variable in color. They range from a pale pink or bright red to a copper orange along the top and bottom. In between on the back half, there's a black triangular or hatchet shaped patch with a bluish tint.Sometimes there's subtle purple highlights in front of the patch as well.

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

This fish is often confused with two of its close relatives, the Glowlight Rasbora Trigonostigma hengeli and the Lambchop Rasbora Trigonostigma espei. To identify them notice some subtle distinctions. The Harlequin Rasbora is pale pink to bright red, and is much stocker than its cousins. Harlequin’s black mark is much closer to a triangle shape and will have a noticeably blue tint. The Espei Rasbora should be bronze pink in color and be more elongate. Hengel’s Rasbora should be yellow cream with a thin black side marking toped with a sweep of neon orange.

These are agreeable fish that do fine in most community settings and even better with other cyprinids. They aren’t aggressive per se but lean towards being a bit boisterous. A school is a must, the companionship of their own species is absolutely essential. You really haven’t seen the Harlequin Rasbora until you’ve seen a school of 10+ individuals in an established tank. These are an excellent schooling fish but the group does need a bit larger aquarium. A 20 gallon tank would be the bare minimum, but they are best suited to a 30 gallon long aquarium (36"+) that will allow them to swim freely.

They are very tolerant fish but do require a little extra care, so are suggested for an aquarist with a bit of experience. They can adapt to a variety of conditions, but optimally they need soft, slightly acidic water and won't do well when kept in hard water. To bring out their best colors use a darker substrate and provide them with plenty of plants. They are very popular in planted aquaria because of how the plants set off their color. The tank should be carefully covered as these fish are liable to jump if startled or excited.

For Information on keeping freshwater fish, see:
Freshwater Aquarium Guide: Aquarium Setup and Care

Geographic Distribution
Trigonostigma heteromorpha
Data provided by FishBase.org
  • Kingdom: Animalia
  • Phylum: Chordata
  • Class: Actinopterygii
  • Order: Cypriniformes
  • Family: Cyprinidae
  • Genus: Trigonostigma
  • Species: heteromorpha
Harlequin Rasboras

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Harlequin Rasboras are a lively and colorful fish.

The Harlequin Rasbora is a fine example of the Cyprinid family of fishes. They make a colorful and lively addition to any community aquarium. A planted aquarium will give them places to dart in and out, as well as set off their fiery color. They are great for people who easily become bored with fish, they swim about constantly and can be a bit boisterous! Not to be kept with shy or slow eating fish, they do well with other lively Cyprinids like tiger barbs and cherry barbs.

Harlequin Rasbora - Quick Aquarium Care
  • Aquarist Experience Level: Intermediate
  • Size of fish - inches: 2.0 inches (5.00 cm)
  • Minimum Tank Size: 20 gal (76 L)
  • Temperament: Peaceful
  • Aquarium Hardiness: Very Hardy
  • Temperature: 73.0 to 82.0° F (22.8 to 27.8° C)
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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. Previously it was also recorded from Indonesia (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 triangular shaped pattern and 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. These are heavily vegetated peaceful waters that are calm or gently flowing. There is often decomposing organic debris, so the water is sometimes stained a yellowish brown by tannins and other chemicals. It is soft and weakly acidic or neutral, and the forest canopies often keep it shaded. These fish swim in schools and are 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 stocker than its cousins and is rather 'tall' with a convex belly area.

Harlequin Rasbora, Trigonostigma heteromorph, Red Rasbora Red Rasbora, Harlequin Photo © Animal-World: Courtesy Barbara Roth

Depending on the region where they are from, 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 - 6 years in the aquarium with good care.

The body can be variable in color. It can range from a pale pink or bright red to a copper orange along the top and bottom. It has a black triangular shaped marking on the back half of its side, parallel to the gills and with a noticeably blue tint, and sometimes there's purple highlights just in front of the patch.

Varieties with a more more copper or golden color are sometimes called the Gold Harlequin Rasbora, those with a black patch and a mostly black body are called the Black Harlequin Rasbora, and those with a strong bluish tinting will sometimes be referred to as the Blue Harlequin Rasbora.

  • Size of fish - inches: 2.0 inches (5.00 cm)
  • Lifespan: 5 years - They have an average lifespan of 5 - 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 conditions, but optimally these fish should have soft, slightly acidic water. They won't do well if kept in hard water, particularly if you want to breed. They are usually not very picky eaters. They will 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

The Harlequin Rasboras are an omnivores and in the wild they feed on small insects, worms, crustaceans, and zooplankton. They should be given a balance of prepared quality flake or pellet food along. This should be supplemented with the occasional live snack like brine 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 - 50% of the tank water should be replaced once a month. If the tank is densely stocked 20 - 25% 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 - It the tank is densely stocked the water changes should be done every other week.

Aquarium Setup

The Harlequin Rasboras are a fairly hardy schooling species that will spend much of their time in open water at the middle and upper regions of the aquarium. A school of 10 or more will need plenty of open room to swim around and a tight cover to prevent jumps. A 20 gallon tank would be the bare minimum, but they will do best in a spacious aquarium of 30 or more gallons, and more importantly in a long aquarium (36+inches) to 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 out help bring out their best colors. Use of hardy floating plants to create shadows will give the fish 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 to their 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 then 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° C)
  • Breeding Temperature: 77.0° F - Breeding temperatures between 77 - 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 for their well being. A school needs to have at least 10 individuals, but more is even better. 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 some less active tank mates can’t tolerate the fast moving lifestyle of the rasboras. However they are small and easily eaten by bigger tank mates. 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 tank mates.
  • Compatible with:
    • Same species - conspecifics: Yes - This fish must be kept with a school of its own kind, need a group of at least 10 individuals, 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
    • Slow Swimmers & Eaters (): Safe
    • Shrimps, Crabs, Snails: Safe - not aggressive
    • Plants: Safe

Sex: Sexual differences

Females are slightly larger and plumper than males. On the males the black triangle is slightly more rounded and the tip will extend further.

Breeding / Reproduction

Breeding the Harlequin Rasbora can be a bit challenging. The 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, their eggs are attached to the undersides of broad-leaved plants or other objects. Pairs need be well conditioned with small offerings of live foods several times a day, usually for about 4 weeks. Conditioning well 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 - 8" (15 - 20 cm). The water should be very soft and not be higher than 1.5 - 2.5° dGH, slightly acidic with a pH of 5.3 - 6.0, and a temperature between 77 - 82.4° 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 too. 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.

Place the pair in the breeding tank, and 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 two will spawn and 6 - 12 eggs will be deposited at a time on the underside of a leaf. About 80 - 100 eggs are typical, though they could deposit as many as 300.

Although they seem to take so much care in depositing the eggs, they seem to forget all of it and 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 - 6 inches (10-15 cm), just make sure the eggs are below the surface.

The fry will hatch in about 24 hours (usually between 18 - 36) and be free-swimming in about 3 - 5 days. The free swimming fry are only 3 - 4 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 disease the are 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. It's advisable to properly clean or quarantine anything that you want add to an established tank prior to introduction, so 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 you deal with it at an early stage. The best way to proactively prevent disease is to give your fish the proper environment and a well balanced diet. The closer to their natural habitat the less stress the fish will have, making them healthier and happy. A stressed fish will is 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 patterns, and these 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.


Author: Barbara Roth, Clarice Brough CFS, Jeremy Roche
Lastest Animal Stories on Harlequin Rasbora

Andrew - 2012-12-30
Ok two mollies The hatchets were not purchased

Andrew - 2012-12-30
hey i have set up a ten gallon tank and i was wondering how many of these little guys i could keep with 2 hatchetfish, 2 Serpae Tetras and a cory? They are all quite small thanx Andrew

  • Jeremy Roche - 2012-12-30
    You can safely add 2 or 3.
Ann mart - 2011-04-01
Top hero son of smilodon is the name of my fish from the title of the great book. Better than harry potter.

Quint - 2010-06-28
They are excellent community fish. I have 8 in my 20 gallon tank along with 12 Zebra Danios, 3 African Butterflyfish, 2 Cobalt Blue dwarf Gouramis, 1 Red-tailed Black Shark, and 1 Siamese Algae Eater. I have seen my rasboras doing the spawning embrace underneath my Amazon sword and Lily Pads. I feed mine brine shrimp, bloodworms, and flake food. Once again, they are excellent community fish, and they school with my zebra danios. I have never seen my zebra danios and my harlequins in 2 separate schools.