The Ultimate Harlequin Rasbora Care Guide for Beginners

Harlequin rasbora (Trigonostigma heteromorpha) is a small, peaceful and good-looking tank fish. Overall, harlequin rasboras are a great choice for community aquariums, adding beauty and activity to the tank with their vibrant colors and peaceful demeanor. Below in this article you’ll find out how to keep it, care about it and about breeding.

Harlequin rasbora (Trigonostigma heteromorpha)

Habitat in the wild

The Trigonostigma heteromorpha belongs to the family Cyprinidae. The family Cyprinidae, commonly known as the carp family, is one of the largest fish families and includes various freshwater fish species, such as carp, barbs, danios, rasboras, and minnows. These fish are found in different parts of the world and are popular in the aquarium hobby due to their diverse and colorful appearances. The harlequin rasbora is a well-known member of this family and is particularly admired for its striking coloration and peaceful nature.

The harlequin rasbora is native to Southeast Asia, specifically found in the countries of Thailand, Malaysia, Singapore, and the Indonesian island of Sumatra. In the wild, these fish inhabit slow-moving waters in heavily vegetated areas, including streams, rivers, and forested swamps. It inhabits streams and other watercourses characterised principally by low mineral content, high concentrations of dissolved humic acids, a consequence of those waters flowing through peat swamp forests.

The waterlogged soils of these forests inhibit the complete decay of leaf litter, and result in the formation of peat, which leaches humic acids and related compounds into the watercourses flowing through these forests. Their natural habitat often consists of sandy or muddy substrates with leaf litter and other natural debris, creating a natural environment for them to forage and find food.

The areas where harlequin rasboras are found are often densely vegetated with aquatic plants and submerged roots. Floating plants and overhanging vegetation provide them with shaded areas and a sense of security. They are usually found in areas with slow to moderate water flow, as they are not strong swimmers and prefer calmer environments.


Harlequin rasboras have a striking and attractive appearance. They have a streamlined, elongated body with a shimmering silver coloration. Each side of body has a velvet violet triangular-shaped mark on it; it starts from the middle of its body and reaches its fluke, resembling a harlequin mask, hence their name. Below the black patch, there is an iridescent orange to red coloration, which adds to their beauty.

How long do harlequin rasboras live?

Harlequin rasboras have an average lifespan of about 3 to 5 years under proper care. However, with excellent water conditions, a balanced diet, and a stress-free environment, some individuals may live slightly longer.

How big do harlequin rasboras get?

Harlequin rasboras are relatively small fish in the aquarium hobby. On average, they grow to about 1.5 to 2 inches (3.8 to 5 cm) in length. Males and females are generally similar in size, with little sexual dimorphism. Their small size makes them suitable for smaller aquariums and allows them to coexist peacefully with a variety of tank mates in a community setup.

Given their modest size, harlequin rasboras are well-suited for nano tanks and planted aquariums, where their vibrant colors and active behavior can add beauty and interest to the overall aquatic environment.

Scientific NameTrigonostigma heteromorpha
Common NameHarlequin Rasbora
Native RegionSoutheast Asia (Thailand, Malaysia, Singapore, Sumatra)
Size1.5 to 2 inches (3.8 to 5 cm)
Lifespan3 to 5 years (with proper care)
Temperature Range72°F to 82°F (22°C to 28°C)
pH Range6.0 to 7.5
Water HardnessSoft to moderately hard
BehaviorPeaceful, shoaling species
Tank SizeMinimum 10 gallons (37.8 liters) for a small group
Tank SetupHeavily planted with hiding spots and open swimming areas
DietOmnivorous; accepts high-quality flakes and pellets, supplemented with live/frozen foods
CompatibilityPeaceful community fish; avoid large/aggressive species
BreedingEgg-scattering species; require a separate breeding tank with dense vegetation
Natural ColorationSilver body with black triangular patch and orange-red lower body
Social BehaviorShould be kept in groups of at least 6 individuals
Natural HabitatSlow-moving waters in heavily vegetated areas, such as streams, rivers, and forested swamps
Harlequin rasbora with violet triangular-shaped mark

Difficulties in keeping

The harlequin rasbora is quite plane and not demanding; therefore it is very spread around the world due to its popularity among aquarists. Though the fish prefers soft and acidic water, its popularity allowed it adapt to different tank water parameters.

Care and keeping in a tank

Tank size

How many harlequin rasboras should be kept together?

Harlequin rasboras are social and shoaling fish, which means they prefer to live in groups. Keeping them in groups is not only more enjoyable for the fish but also beneficial for their well-being. A group of harlequin rasboras provides a sense of security and reduces stress, making them more active and colorful.

The tank size for harlequin rasboras depends on the number of fish you plan to keep and whether you want to create a community aquarium with other fish species. Since harlequin rasboras are relatively small and peaceful fish, they can thrive in a variety of tank sizes.

If you have chosen harlequin rasbora as a “main” fish in your tank, buy a school of at least 10 species – this way the fish will show its best sides in a tank. For the fish to feel comfortable in a tank, choose the tank volume considering that 1 fish requires 5 liters (1,32 gal).

So, taking into account small number of tank mates, tank bottom substrate and decorations, a minimum tank size of 10 to 15 gallons (37.8 to 56.8 liters) is suitable. This size provides enough swimming space and allows for proper filtration and maintenance.

As for the tank length-width-height – these are not important parameters for the fish, however, taking into account live tank plants which I do recommend to put into tank, than its width and height are better to be equal to 40-45 cm (15,75-17,72 in) and its length depends on the tank capacity you have chosen.

If you want to keep a larger group or create a community tank with other compatible fish, consider a larger aquarium. For example, a 20-gallon (75.7 liters) tank or larger would be ideal for a community setup with harlequin rasboras and other peaceful fish.

Tank decor

It’s recommended to keep harlequin rasbora in planted tanks with plants belonging mainly to South Asia region.

Several kinds of simple cryptocryne will do quite well. On the background and in the tank corners you can put some large plants like Cryptocoryne Wendtii var. «Brown» or/and Cryptocoryne Usteriana, in the middle distance/in the centre there may be Cryptocoryne lutea.

Small amount of fluctuant in the tank is a plus, for example, water lettuce; the fish feels more comfortable in its presence.

As for the bottom substrate for tank – it’s desirable to be a dark one, since the fish look better and more natural with such background, and they are less stressed as well.

Since we’ve chosen large kinds of Cryptocoryne, then the plants layer near the back tank wall will be about 10 cm thick (4 in) and to create some perspective effect let’s make a small incline to the front tank wall, where the plants layer will be about 2-3 cm (1 in) thick.

Water parameters

To keep harlequin rasboras healthy and thriving in your aquarium, it’s essential to maintain specific water parameters that mimic their natural habitat in Southeast Asia. Here are the recommended water parameters:

  1. Temperature: 72°F to 82°F (22°C to 28°C). Aim for a stable temperature within this range, as fluctuations can stress the fish.
  2. pH: Slightly acidic to neutral. The ideal pH range for Harlequin Rasboras is around 6.0 to 7.5. Keep the pH as stable as possible.
  3. Water Hardness: Soft to moderately hard. Aim for a water hardness (GH) level between 5 to 12 degrees, and carbonate hardness (KH) between 2 to 10 degrees. Water in harlequin rasbora habitat is soft, but considering the fact that the fish has been popular in aquarium husbandry for quite a while, then their tank water chemical composition has no crucial significance. This is true when we talk about the fish keeping in a tank, but in case of the breeding you’ll have to pay more attention to the tank water composition.
  4. Ammonia and Nitrite: These should be at 0 ppm (parts per million). Ammonia and nitrite are toxic to fish, so it’s crucial to maintain a well-cycled aquarium with effective biological filtration.
  5. Nitrate: Keep nitrate levels low, ideally below 20 ppm. Regular water changes help control nitrate buildup.
  6. Dissolved Oxygen: Ensure adequate aeration and surface movement to maintain proper oxygen levels in the water. Filtration in tank is a must; the fish likes the presence of light flow there. As for the tank aeration, it’s not necessary since there are a lot of live plants in it.

Regular water testing using a reliable test kit will help you monitor and maintain the water parameters within the appropriate range. Performing regular water changes (about 20-30% every 1-2 weeks) will help keep the water quality high and provide a healthy environment for your harlequin rasboras. Additionally, using a high-quality aquarium filter and maintaining good overall tank hygiene will contribute to the well-being of your fish.

Tank mates

Harlequin rasboras (Trigonostigma heteromorpha) are peaceful and social fish that can coexist with a wide range of tank mates in a community aquarium. Since the harlequin rasbora is rather peaceful, you can keep it with any fish as long it’s not large and not a predator.

For example:

Avoid keeping harlequin rasboras with larger or aggressive fish that may intimidate or prey on them. Also, try to avoid fin-nipping fish as they may harass the harlequin rasboras.

Juveniles are timid and they prefer to stay in the shadow of the tank plants. When growing old they start showing more interesting behavior. If harlequin rasbora juveniles either swim in a big school, or drift away around the tank to find something to eat; when getting older they try to get more honorable according to the school hierarchy place and attract the attention of the opposite gender species.

To find out who is the best males get frozen side by side at a distance of about 2 cm from each other and then start to go in circles spreading their fins wide and trembling a little at the same time. At that not only the male can do this, the female ones also can but more seldom.

Such fights are bloodless, although sometimes the fish make rather sudden moves, still it doesn’t come to any injuries. As a rule, after some time the rivals agree to a draw or the weakest one cedes and abandons the “field”.

Harlequin rasboras in a big school


Harlequin rasboras are omnivorous fish, meaning they eat a varied diet that includes both plant matter and small animal-based foods. In the wild, they feed on small insects, zooplankton, algae, and detritus. In an aquarium setting, you can provide them with a balanced diet to ensure their optimal health and vibrant coloration.

Here are some suitable foods for harlequin rasboras:

  1. High-quality Flake or Pellet Food: A good quality flake or pellet food designed for tropical fish can serve as the staple diet. Look for fish foods that contain a mix of proteins, carbohydrates, vitamins, and minerals. Specialized foods like micro pellets, small granules, or crushed flakes are available, specifically designed for small fish like rasboras. These are suitable options for feeding them.
  2. Live or Frozen Foods: Supplement their diet with live or frozen foods to mimic their natural diet and provide essential nutrients. Brine shrimp, daphnia, bloodworms, and microworms are excellent options.

It’s essential to feed harlequin rasboras small amounts multiple times a day, rather than a large meal once a day. This feeding approach better reflects their natural feeding behavior and helps prevent overeating or food wastage.

As with any fish, always ensure the food is of high quality, and avoid overfeeding to maintain good water quality and prevent health issues.

Gender differences: male vs female

Distinguishing between male and female harlequin rasboras can be a bit challenging, as they do not display prominent sexual dimorphism (physical differences between males and females). However, there are a few subtle characteristics that may help you differentiate between the two:

  1. Size: In some cases, female may be slightly larger and rounder than males, especially when they are carrying eggs. You can see difference between harlequin rasbora female and male since the female has more rounded abdomen.
  2. Coloration: This is a tricky one, as both males and females typically have the same color pattern. The male is more graceful and brighter colored.
  3. As for their black triangular spot on the body sides, the male have more sharpened mark; this mark on the female body is rounded.
  4. Behavior: During breeding, males may become more territorial and exhibit some chasing behavior as they try to court females. However, this behavior might not be exclusive to males, as females can be active participants in the breeding process.
Harlequin rasbora two males


When harlequin rasbora breeding they become rather demanding as for their pair selection and the surrounding environment.

The fish breeds in long tanks where the water layer shouldn’t exceed 20 cm (8 in); at dim light and in soft water with the temperature 24-28°C (75,2-82,4 F).

Cryptocoryne or Java fern fixed on the tank bottom with some glass stick may be used as spawning substrate.

As a rule in the evening we put 2 males and 3-4 harlequin rasbora females into a spawning tank planted with Cryptocoryne, seedbox or water hawthorn. While spawning the female fish usually plays an active part.

She tries to make the male swim to the water surface, where both fish cuddle together and turn their abdomens up, and the female sticks several eggs on the bottom side of some tank plant leave.

The female does this until she lays all the eggs. After that the male and female are taken away from the tank.

From dozens of eggs only several harlequin rasbora juveniles can be obtained. Special attention should be paid to the tank water temperature – it should constantly be about 28 °C (82,4 F), since the eggs are incredibly sensitive to all kinds of water temperature change.

About 1.5 days after the spawning the larvae appear, which become juveniles in 5 days. The juveniles are fed with cyclops nauplii, infusorians.