Harlequin Rasboras – Detailed Guide: Care, Diet, and Breeding

Harlequin Rasboras (Trigonostigma heteromorpha) – Detailed Guide Care, Diet, and Breeding

Harlequin Rasbora is a highly sought-after species of freshwater fish owing to its vibrant colorations and splendid interactive behaviors.

This small-bodied fish is perfect for beginners since Harlequin Rasbora is relatively easy to care for and will survive in non-pristine water conditions. 

Keep reading for everything there is to know about the Harlequin Rasboras, and how to nurture the species in a home aquarium.

Quick Notes about Harlequin Rasboras

Name Harlequin Rasbora
Other Names Harlequin fish
Scientific Name Trigonostigma heteromorpha
Tank size (minimum) 10 gallons (~40 liters)
Keeping Easy 
Breeding Difficult
Size 1.5 – 2 inches (~ 3.5 – 5 cm)
Optimal Temperature 72 – 81 °F (22 – 28 °C)
Optimal PH 6.0 – 7.5 
Optimal GH 2 – 15   
Optimal KH 1 – 12
Dwellers Top and middle
Nitrate Less than 40
Diet Carnivore / omnivore
Temperament Peaceful
Life span up to 6 years
Color Form Reddish, pinkish, or orange body with a wedge-shaped marking

Etymology and Origin of Harlequin Rasboras

The “Harlequin Rasbora” (Trigonostigma heteromorpha as its binomial name) is a fish introduced to the hobby in the 1900s, since then they have been hugely popular thanks to their small size easy-going nature, and lovely coloration.

As a miniature fish, it was first categorized into the genus “Rasbora” and named “Heteromopha,” a Greek word ‘Trigonon’ that means “differently shaped.” This naming is befitting because its body shape deviated significantly from other members of the genus.

The name “Harlequin” then came about because the fish possessed a black triangular patch on its body that resembled patterns found on a clown’s costume.

This species is classified as follows:

Kingdom: Animalia
Phylum: Chordata
Class: Actinopterygii
Order: Cypriniformes
Family: Cyprinidae
Subfamily: Danioninae
Genus: Trigonostigma
Species: Trigonostigma heteromorpha

Habitat of Harlequin Rasboras

It is commonly found in Malaysia, Singapore, and Thailand.

Being a native of Asian regions, the Harlequin Rasbora resides in the lowland waters and eschewing brackish waters. These lowland waters, such as streams, swamp forests, and rivers, contain low mineral content, high acids, and soft concentrations.

Acids are prevalent in the water because the water flows through swamp forests. The damp soils of these forests impede the thorough decay of leaf litter and result in the building of peat, which leaches tannins, humic acids, and similar compounds into the watercourses streaming through these forests.

Harlequin Rasbora enjoys living in an environment with dense foliage, ample room for swimming; featuring dark substrates, slow to moderate water flow, and reduced illumination.

Description of Harlequin Rasboras

Harlequin Rasboras (Trigonostigma heteromorpha) – Detailed Guide Care, Diet, and Breeding - in planted tankIt has been well documented that the Harlequin Rasbora is small in size. At 1.5 to 2 inches, (3.5 – 5 cm) they can fit into most tanks easily.

Note: Personally, I have never seen them grow up to 2 inches. My Harlequin Rasbora barely reach 1.7 inches or 4 cm in length.

  • Their color pattern consists in a reddish, pinkish, or orange body. The bellies are usually whitish.
  • They have a forked caudal fin and a black patch that forms narrowly along the body and ends as it gets to the caudal fin.
  • Absence of barbels.
  • Every Harlequin Rasbora has a wedge-shaped marking. The shape begins at the dorsal fin and tapers to a fine point at the base of the tail. The rest of the body doesn’t have any marks, creating a very staunch distinction between both sides.
  • The fins can vary from dark to light-orange intensity and are affected by environmental conditions and stress.

Worn-out harlequins will have transparent pale orange fins that will slowly darken as they begin to get more comfortable.

To maintain the healthy appearance of a Harlequin Rasboras fish, you should provide ideal conditions.

Lifespan of Harlequin Rasboras

The typical lifespan of the Harlequin Rasbora is between 3 and 5 years.

Some will live until about 6-7 years old, while proper care and genetics play a huge role in keeping them alive to the upper end of that range.

These fish are significantly affected by water conditions and the overall quality of their habitat.

Typical Behaviour of Harlequin Rasboras

  • Temper:

 Harlequin Rasboras are feeble and gentle fish who will not trouble other tank mates.

If anything, they are more likely to be on the receiving end of aggression as their small size places a target on their backs from other bigger fishes.

  • Sociality:

Harlequin Rasboras (Trigonostigma heteromorpha) – Detailed Guide Care, Diet, and Breeding - in big numbersHarlequin Rasboras usually spend time with their other mates, scouring their environment. For them, strength lies in numbers.

They can live peacefully with other fishes, but they quickly become stressed if they do not have a vast shoal of their species. They generally stay in a very tight group. However, once they become used to a tank, they may start swimming individually from time to time.

If this species is housed in a fish tank, it should be with only smaller species. A group of 6-8 is an excellent number to stock your tank with.

  • Activity:

Harlequin Rasboras are jumpers. So, you need to have a tight lid and/or keep that water level at least a few inches below the rim of the tank.

They are always on the move but they do not rush from one end of the tank to another.

  • Temper:

Harlequin Rasboras are not very timid fish. On the contrary, they are pretty active and outgoing but only under some very important conditions:

a) There must be some cover for them to dash into if they feel threatened. In a heavily planted tank, for the most part, you will see them schooling around in the open.

b) They should be kept in numbers.

c) No big and/or aggressive fish around.

  • Placement in Tank:

While the fishes might wander off for a good number of times, they still stay together and wade throughout the middle and top part of the tank.


  • Social: Yes
  • Activity: Average
  • Placement: Middle and top dwellers
  • Peaceful: Yes
  • Jumpers: Yes

Feeding Harlequin Rasboras

Harlequin Rasboras is an omnivore but prefers eating live food. They are classed as micro predators or micro-carnivores.

These small fish are not picky with food.  They enjoy consuming frozen, pellet, granule, or flake foods. Actually, if you diversify their diet they will be healthier and better colored.

Such a variety in their diet is nutritional and will build their digestive system’s immunity against diseases.

Note: Harlequin Rasboras do not eat algae or blanched vegetables.

Due to their tiny mouths, they only eat tiny particles of food. You can crush them up between your fingers.

You can feed them a wide variety of meals such as:

Harlequin Rasboras are not shy at feeding. Even more, they are overly zealous about food! They dart up to catch the food as they sink. In some cases, they can even jump out of the tank. So, be careful.

How Often and How Much to Feed Harlequin Rasboras?

  1. Harlequin Rasboras should be fed at least 5-6 times per week. 
  2. The food should be gone pretty quickly (within 1-2 minutes). It should not reach the bottom. If it gets to the bottom, you are giving them too much.
Important: Remember – it is better to underfeed than overfeed. Overfeeding is a major problem in maintaining water quality.

Are Harlequin Rasboras Plant Safe?

Yes, Harlequin Rasboras are plant safe. They do not eat live plants.

Even more, plants are needed for them especially for breeding purposes.

Keeping and Caring for Harlequin Rasboras

The Harlequin Rasbora is relatively hardy and resilient and doesn’t get sick often.

However, if it is not cared for properly, it can contract white spots on its body (ich), fluid build-up (dropsy), and fin rot. All these infections thrive in tanks with dirty water and unstable conditions.

It is important that you cycle your tank before bringing fish home. Once the tank is cycled you need to check the quality of the water using a test kit.

Also, cleaning up your tank is a straightforward preventive measure to ensure you don’t endanger your fish.

Tank size:

The tank should be a standard rectangular size with ample surface area. At least 10 gallons (40 liters) for a small number of 6-8.

Note: Some aquarists say that Harlequin Rasboras are flexible enough to excel even in a nano setup. Personally, I am against this practice. 10 gallons is a minimum requirement.

If you have more space, larger tanks of 20-30 gallons (80-120 liters) will be more appropriate, as it permits increased stock and provides more room for the fish to explore.

For breeding purposes, you should get a large fish tank to replicate the natural habitat to breed a Harlequin Rasbora domestically.

Important: You need to get a lid for the tank, Harlequin Rasboras are good jumpers. 

Related article:

Water parameters:

Temperature: You must ensure that the water in the tank is warm 72 – 81 °F (22 – 28 °C). This is because Harlequin Rasboras live in such conditions since they are tropical species. If needed, you can achieve this by adding a heater to the tank.

pH: Ideally, ensure the pH level is around 6.0 – 7.5. However, Harlequin Rasboras are very adaptable and can tolerate pH between 5.5 – 8.0.

Note: In the wild, this species is found in really acidic water type conditions. Black water is mostly required for breeding purposes.

Hardness:  Keep the hardness level at 2 – 15 GH.

You must always test for the water parameters and regularly replace about 25% of the volume. This activity ensures the fish stays healthy and prevents it from being stressed.


No species requirements.

However, if you are planning to breed Harlequin Rasbora, I would use sponge filters for that.

These filters are baby safe; the babies will not get sucked into that. With most other filters, you will need an extra pre-filter in the form of a sponge to prevent fry from being sucked in.


The substrate should be adapted to the needs of plants in your tank. Also, substrate can play a crucial role if you need to keep your pH less than 7.0 (neutral).

For example, active substrates (such as ADA Amazonia aqua soil, Fluval Plant and Shrimp StratumAkadama-Bonsai soil, etc.) will lower the water parameter (pH).

Tip: The black substrate is very important to bring out intense coloration. At the same time, if you put Harlequin Rasboras in the tank with a white substrate and lots of light, the color fades away.


The lighting should be also adapted to the needs of plants in your tank. Ideally, you need to pick floating plants or plants that do not require a lot of light.

Too much light in the Harlequin Rasboras tank washes out their coloration.

It can sound strange but by doing so they try to camouflage from predators. Camouflage is considered the primary function of adaptation to different habitats and background, rendering the animal cryptic to potential predators.

Water flow:

Although Harlequin Rasbora can easily adapt to any water flow in the aquarium, they typically prefer slow to moderate currents. That is because their home habitats tend to be boggy and prone to periods of both stagnation and flooding.

Plants and Decorations:

Also, there should be live plants in the midground and corners of the aquarium. One of which includes Cryptocoryne spp.

This plant grows in the same environment Harlequin Rasboras reside. They are soft enough to swim through, and with their broad leaves, offer security to the fish.

The plants will take up excess nitrates from the aquarium water, thus keeping it clean and less harmful.


Before putting Harlequin Rasboras into your tank do not forget to carefully temperature acclimate them.

Do not rush the process! Do it very slowly to prevent any unnecessary stress.

Harlequin Rasboras – Male and Female Differences

Harlequin Rasboras (Trigonostigma heteromorpha) – Male and Female DifferencesMale and female Harlequin Rasboras differ in appearance. However, it can be really hard to see in most cases.

  • Size. The males are a little bit smaller and slender, with their female counterparts being bigger.
  • Shape of the triangle. Females also have a rounded extension at the lowest edge of the distinct black wedge covering the tail, while the males have a straight wedge.

Breeding Harlequin Rasboras

Although it is very easy to care for Harlequin Rasbora, it is not an easy task to breed them yourself because it requires a certain level of expertise.

As a matter of fact, this species is still heavily traded in the ornamental fish trade based mainly on the wild-caught stock. However, several selectively bred fish are also available.

–         Breeding Conditions:

You must ensure that their living condition mirrors the outside world during the breeding season.

  1. First, build a separate breeding tank with equal water holding capacity as large as the former. Of course, it is possible to breed them in the main tank but Harlequin Rasbora may eat their eggs and fry.
  2. The fishes need very warm temperatures to breed, between 77 – 83 °F (25 – 28 °C), so heat it.
  3. Low pH. Also, unless the pH level of the water is around 6.0 – 6.5, you will not see any eggs. Therefore, if your pH is above those parameters, you will have to lower it down. For example, you can filter it over peat, add more dry leaves, cones, and driftwood, thus, imitating the humic acid concentrations present in the fish’s native waters.
  4. Soft water. Your GH should be around 1 – 5.
  5. Ensure that broadleaf plants are present in the tank, as the female Rasboras will use it to lay their eggs.
  6. Feed them protein-rich food (tubifex worms, blackworms, daphnia, brine shrimp, etc.).
  7. In the evening, a group of fish (2 males and 3-4 females) should be moved to the breeding tank and left for 1-2 days. Spawning usually occurs early in the morning.

–         Mating:

It is pertinent to place the Rasboras in the tank in a technical manner, i.e., one female for every male. This technique increases the chances of breeding.

Note: Some aquarists recommend adding a few pairs at the same time. Personally, I found this counterproductive because they do not mate at the same time. When they are not ‘busy’, they may go after the eggs.

The males display in front of the female and nudge their body parts. An action that directs the females to the ‘spawning location.’

When the females are ready for fertilization, they turn upside down, stroke their bellies against the bottom of the leaf, and beckon on the male. Then, he fastens around her and fertilizes the eggs as she discharges them.

–         Eggs

Harlequin Rasboras are not livebearers, they are egg layers. However, unlike other Rasbora species, they are not egg scatterers.

The eggs are spawned on the lower surface of large submerged leaves or similar structures. The eggs are very adhesive and generally do not fall on the bottom.

Note: Eggs lose their adhesiveness in hard water. Thus, if you see them falling, you need to look into this problem.

About 6 to 12 eggs are released at once. The eggs stick to the base of plant leaves. This process occurs continually, and about 100-300 new eggs are spawned in an hour or two.

Once the spawning is finalized, you must remove the adult fish from this area, as they can eat up the offspring.

Some potential problems: If the water is too hard, the eggs may completely dissolve. The eggs may also turn whitish. This usually happens when the eggs are overripe, or the male is not fast enough to fertilize them.

–         Hatching

After 24 hours, fry emerges from the eggs – a new life is formed. The fry is rather large and yellowish. The next day they usually do not move much and stay on the plant leaves or lie on the bottom.

They then begin to swim freely, acclimatizing themselves to their new environment.

Feed them with fine infusoria, moina, and brine shrimp for 14 days, after which you can transition to powdered fish food.

After two weeks, dark specks appear on their sides and a black rim at the base of the caudal fin. After another 10 days, the fry can eat daphnia.

Once the fry is big enough not to be eaten, it can be added to the community tank.

In about 6-9 months, they reach full maturity.

Harlequin Rasboras and Suitable Tankmates

Harlequin Rasboras are generally peaceful fish, making it easy for you to find them tankmates. Harlequin Rasboras should be kept together. As they are a shoaling species, keeping 8 to 10 of them in the same space is a good idea, as long as there is ample space for swimming. The larger their group, the more glamorous their display.

–       Fish

Basically, these small fish will be happy with almost any fish that is not big enough to eat or harass them.

Some compatible fish tank mates are Pygmy Cory, Chili Rasboras, Panda Garra, Albino Bristlenose Pleco, Neon Tetras, Brilliant Rummynose Tetras, Zebra Danio, Ruby Tetras, Dwarf Ember Barbs, Clown Killifish, Dwarf cichlids (Apistogramma agassizii), Guppies, Endlers, Medaka Ricefish, Ghost Glass Catfish, Southern platyfish, Yunnanilus cruciatus, etc. However, it is in your best interest to monitor their interaction to ensure nothing goes haywire.

Species like Cichlids, Common Pleco, Monos & Scats, Jack Dempsey, and other boisterous fish can be pretty unfriendly to them.

–       Dwarf shrimp

For diversity purposes, one can add other invertebrates like dwarf shrimp. They are also capable of cleaning up after the Harlequin Rasboras.

However, if you are planning to breed dwarf shrimp it will not be a good decision. Harlequin Rasboras will not harm adult shrimp but they will definitely feed on shrimplets

–       Snails:

Harlequin Rasboras are compatible with any freshwater snail. They will do a good job in scavenging for food particles, plant matter, and algae along with the lower levels of the tank.


In Conclusion

Harlequin Rasbora is a popular fish species that cost less than $5 per specimen. In addition, it is a small, peaceful species ideal for hobbyists that fancy an undemanding, attractive shoaling fish.

If you are careful and capable enough to provide them with a suitable, healthy environment, then they will make a perfect shoal for your viewing pleasure.

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