Harlequin Rasbora 101: Care, Diet, Tank Size, Tank Mates, & More

Ever noticed how almost all of the home aquariums you see have a Harlequin Rasbora? 

It is aquarists’ all-time favorite freshwater fish because of its shimmering appearance. Watching this colorful fish moving around the aquarium never gets old. But that’s not all. This fish has a peaceful temperament and is easy to care for.

Read more if you want to add a splash of color without any violence in your tank.

Harlequin Rasbora
Harlequin Rasbora

Species Summary

The Harlequin Rasbora (Trigonostigma Heteromorpha) is a tropical fish belonging to the Cyprinidae family. It has been present in aquariums since the early 1900s (more precisely, 1906) when the first harlequin fish arrived in aquariums.

German ichthyologist Georg Duncker, the curator at the Hamburg Zoological Museum, discovered the species in a pond at the Singapore Botanical Gardens, describing the Harlequin Rasbora in 1904 in a review paper on freshwater fish collected in the Malay Peninsula.

The fish can vary slightly depending on the collection locality. Those collected in Johor and southern Thailand have a minor appearance, giving a thinner set than Singapore and Sunda Island populations. Furthermore, all Harlequin Rasboras collected in the next 20 years arrived in Europe directly from or via Singapore. However, the type locality is cited as “Kuala Lumpur, Selangor, Malaysia.”

Thai populations appear to be restricted to the Narathiwat region, close to the border with Peninsular Malaysia. It is more widely distributed and collected in forested areas of Selangor, Kelantan, Terengganu, Pahang, and Johor states in Malaysia.

In Singapore, it is found in the Nee Soon swamps, one of the last pockets of pristine freshwater swamp forests in the island’s center.

The extent of its spread in the Sunda Islands and beyond is less clear. While it indeed occurs on the island of Bintan in the Riau Islands province (Sumatra), it is impossible to find a definitive conformation on the mainland other than a vague reference to the city of Medan in the north (the species is said to cover all of northern Sumatra). Likewise, there doesn’t seem to be a single confirmed sighting in Borneo.

There are different varieties of these harlequin fish: Golden (xanthic), Black (melanic), and Blue. Different studies have confirmed that the harlequin fish species available for sale have morphological inconsistencies due to crossing different varieties.

Finally, its growing popularity is because it is disease-resistant and friendly. This fish is able to share space with a wide variety of different species; it is one of the most suitable for beginner aquarists.

Harlequin Rasbora Care Guide

The Harlequin Rasbora is a beautiful fish with evident ornamental characteristics due to its colors. Learn more about its tank size requirement, tank mates, water parameters, diet, and more in this section of the guide.

Tank Size

As it is an active species and is found in large schools, regardless of its size, it should be kept in an aquarium of considerable size. Dimensions of 24″ X 11″ X 11″ (12 gallons) should be taken as the minimum size for a small school of these lush fish.

Like other schooling fish, the more space available for them to swim freely, the larger the school, and the more beautiful their coloring and natural behavior.

As a general rule, if you want to keep them in community aquariums, you must expand this minimum size concerning the other species you want to keep.

Tank Mates

These Rasboras are a very calm species with no problems with any type of fish except those that can eat them due to their size.

They are gregarious, so keeping a small Harlequin Rasbora community is advisable. We always recommend keeping between 8 and 10 individuals to achieve a more natural effect; this way, the males show their best colors and health.

In a community aquarium, they can share space with Cory Catfish, Dwarf Gourami, Danios, Tetras, small Cichlids, and even some Betta varieties.

Same Species Tanks

In the same way as other Rasboras, the Harlequin is widely used as a single species to compose different landscapes in the aquarium, mainly in biotope aquariums or in blackwater tanks.

Water Parameters

The waters they inhabit are warm, soft, and acidic as they are fish from forested regions.

The species is found in small, shaded forest streams in the wild, usually with moderate flow. The water is sometimes slightly tinted a yellowish brown due to the presence of tannins and other compounds released by decomposing organic matter and a substrate littered with dead leaves and wood.

The low lighting of these environments is due to the dense vegetation on the banks and the dense tree cover above the small streams.

Keep the water soft (GH < 5, if possible < 3), with a temperature between 23 and 27°C and a slightly acidic pH (5.0 to 7.0).

What to Put in Their Tank

Give preference to dark-colored substrates; this way, you can recreate the natural habitat of this species. The dark color will also enhance the natural color of the fish. This effect can be achieved using a sandy substrate, covering it with dead leaves, peat, or coconut fiber.

Driftwood and roots are welcome. They form holes, caves, and shaded areas, as well as providing Harlequin Rasboras with hiding places. In addition to giving a greater sense of naturalness, these natural decorations, such as dry leaves and driftwood, when they start to decompose, will serve as food for the fry and help to maintain the acidic pH.

The aquarium lighting and the water flow inside the tank should be dim. Regarding filtration, they prefer less agitated water, reducing the aquarium filter’s flow rate.

They also like densely planted aquariums, with plants forming shady areas. They are more colorful and active when kept in a planted aquarium with open areas for swimming.

You can use plants like Cryptocoryne and Microsorum, Java Moss of the Taxiphyllum genus, or even sword plants like Echinodorus. Offer a few patches of floating vegetation to spread the light, which can be effective for breeding.

Common Diseases

Some aquarists have problems with diseases like Fin Rot, Ich, and Dropsy, which are directly caused by poor quality of water in the aquarium. Diseases can also affect fish due to other environmental stresses, such as too much light or lousy handling.

With consistent excellent water quality, these Rasboras will never present any problems.

Food and Diet

It is an omnivorous fish, which in its natural habitat, feeds on small insects that fall into the water, worms, crustaceans, and zooplankton. Being a micro predator, it does not present any difficulty in feeding.

In the aquarium, it will readily accept any food. 

To maintain a balanced diet, you must provide high-quality commercial foods supplemented by dry, live, and frozen foods such as earthworms, bloodworms, Daphnia, and brine shrimp. Although live foods are an essential part of the diet of these animals, try not to overdo it, as they can start to refuse commercial foods.

These fish live best when they eat several times a day, but in small amounts that they can consume within 3 minutes.


These Rasboras live for about 3 years in captivity. In the above-ideal conditions, they can live a little longer, reaching close to 4 years.


This species has a stocky pink body with red tips and a tall, diamond-shaped body, which sets it apart from other Rasboras species.

The fins are transparent with a slight shade of red and black. Its large black triangular patch on the back of the body is the most recognized feature. This patch can become very light or even almost white during periods of stress.

The coloring on the fins’ head, back, and end are intense. On the belly side, the fish appear silvery to white.


There are records of specimens collected and kept in aquariums up to 1.9 inches in length. However, the size we usually find these fish is around 1.18 inches.

Due to its small size, it is ideal for planted aquariums or ornamental shrimp.

Behavior and Temperament

Very peaceful, friendly, and gregarious, it should be kept in a large group, if possible, at least 10 to 12 individuals. Harlequin Rasbora lives in simple structured social organizations with a slight hierarchy (free hierarchy).

Males will show off their colors as they compete for the females’ attention. You might even witness some chases between them without real consequences.

And the best part is these animals are not difficult to care for as long as the aquarium meets their specific needs. Also, they will spend most of their time in the middle or upper regions of the aquarium.


These Rasboras are oviparous; laying eggs is done differently from other cyprinids. 

This species usually lays its eggs on the underside of plants with broad leaves or decorations; instead of scattering the eggs randomly. Spawning takes place in shallow water 15 to 20 cm deep.

The idea is to set up a specific aquarium for reproduction, which is poorly lit and has a shallow water level of about 5.9 – 7.8 inches. In addition, the water should be soft, and the hardness should be between 1.5 and 2.5.

There is no need for filtration in the aquarium, but they need broadleaf plants like Java Fern and Cryptocoryne to lay eggs.

After moving the couple to the breeding tank, spawning will occur in the morning. The male will “take” the female to the chosen location, almost always the underside of a plant. Then, he will dance, and the spawn will follow soon after, with the two swimming upsides down.

Males perform courtship displays for breeding, with fins widely open and spread apart, also display intense coloration and offer sudden movements around breeding females. 

They form pairs of short duration, precisely at the time of reproduction. The females glue the eggs to the underside of the leaves of aquatic plants, and fertilization by the male occurs shortly afterwards.

Once spawning is done, remove the parents and leave the aquarium very dark. Although they are meticulous in laying eggs, they will still eat them if they are not removed from the aquarium after the procedure.

The eggs hatch between 24 and 48 hours, and the fry will be free swimming after about a week. They reach sexual maturity at about 10-12 months.

Fingerlings are no more than 4 mm long, so provide them with infusoria.

Like other cyprinids, the Harlequin Rasbora does not have any parental care for the young; separating the parents from their eggs is advisable.

Gender Differences: Male vs Female

Sexual dimorphism is expressed in the male by a bluish-black spot much more rounded than in the female and a slightly more pointed and tapered dorsal fin.

What’s more, females are slightly larger and plumper than males, especially in the ventral region. Adult males have straight bodies and are slightly more colorful.

Harlequin Rasbora Fun Facts

  • The Harlequin Rasbora Trigonostigma heteromorpha is still known by its ancient taxon, Rasbora heteromorpha or by the common name harlequin heteromorph fish. 
  • Native to Southeast Asia, mainly Peninsular Malaysia and Thailand, the harlequin fish, is also present in Singapore. Their Thai populations are threatened, and records from Indonesia (Sumatra) refer to another species, T. hengeli.
  • Due to their incredible popularity, breeders bred the fish en masse to supply the aquarium market, which resulted in morphological deformations in some situations and a decrease in color compared to wild specimens. 
  • Several ornamental strains selectively bred have also become available, including the blue, black, and gold forms, whose care is identical to the wild form.
  • It can be confused with T. hengeli and T. espei, although they are straightforward to differentiate on closer inspection. The genus was created by Kottelat and Witte (1999) to separate members of the Rasboras grouping.
  • Rasbora has been recognized as a polyphyletic lineage, as noted by Kottelat (1999), among others, and in 2010 the results of a phylogenetic analysis by Liao et al. suggested several changes to improve the taxonomy. 
  • The authors found species of Rasbora genera to represent an existing monophyletic grouping in six clades and created four new genera containing former members of the genus Rasbora to preserve the monophyly of the current groups.
  • The first two clades contain new groupings, Kottelatia and Brevibora, respectively. The third comprises Boraras, Horadandia, Rasboroides, and Trigonostigma, in addition to the new genera Trigonopoma and Rasbosoma. However, the results for Boraras and Trigonostigma were inconclusive in some respects, and scientists recommended further work on their phylogenetic position. Soon after, an article researching the systematics of the subfamily Danioninae was published (Tang et al. 2010). Results differed significantly, and the four new genera by Liao et al., in addition to Boraras and Trigonostigma, were synonymized with Rasbora, an approach described as “more conservative.”


Kottelat, M. and K.E. Witte, 1999. Two new species of Microrasborafrom Thailand and Myanmar, with two new generic names for small Southeast Asian cyprinid fishes (Teleostei: Cyprinidae). J. South Asian Nat. Hist.

Lim, K.K.P. and P.K.L. Ng, 1990. The freshwater fishes of Singapore. Singapore Science Centre, Singapore.

Monkolprasit, S., S. Sontirat, S. Vimollohakarn and T. Songsirikul, 1997. Checklist of Fishes in Thailand. Office of Environmental Policy and Planning, Bangkok, Thailand.