Harlequin Rasbora – Size, Feeding, Lifespan, Tank Mates and Care Guide

Harlequin Rasbora

Harlequin Rasbora (Trigonostigma heteromorpha) is a small freshwater fish from Southeast Asia. They are from the streams of Singapore, Malaysia, and Thailand. The one from Sumatra island, Indonesia refers to another species.

Harlequin Rasboras are a great shoaling fish to add to your community tank. Harlequin Rasbora is the most popular of over 60 species of Rasboras. They are also known as Red Rasbora, Harlequin fish. 

Let us continue reading to know more details about the aquarium hobbyists’ favorite Rasbora, Harlequin Rasbora.

An Overview – Harlequin Rasbora:

Description Category
Family Cyprinidae
Scientific name Trigonostigma heteromorpha
Origin Southeast Asia
Type Fresh Water
Color Form Reddish-bronze body with black wedge
Difficulty Easy
Breeding Moderate
Max. Size  inch (5 cms)
Life span 5 to 8 years
Temperament Peaceful
Diet Omnivores
Minimum Tank Size 10 gallon ( liters)
Temperature 70 – 82°F (21 – 28 °C)
Acidity 5 to 7 pH
Hardness 5 to 12 dH

Species Origin:

Harlequin Rasbora is from the streams of Singapore, Malaysia, and Thailand. It is a member of the Cyprinidae family. The species was introduced into the aquarium hobby in the early 1900s by the traders. It became instantly popular among hobbyists. 

William Thornton Innes III, an aquarist and author of innumerable influential books and articles on aquarium fish, aquatic plants, and aquarium maintenance, has graced the cover of the first edition of his publication, and the image of the trio of harlequin remained through all 19 editions.

Its scientific name Trigonostigma heteromorpha, meaning “differently shaped rasbora” in Greek is to imply to its body shape is different from other members of the genus.

Its common name, harlequin rasbora implies, the black wedge patterns in the second half of the body. 

Harlequin Rasbora is a peaceful freshwater small tropical fish. 

The species is near to its extinct in its natural habitat due to the expansion of the agricultural activities of these regions.

Harlequin Rasbora – Physical Characteristics and Appearances:


Harlequin Rasbora is a freshwater fish. They grow to approximately 2 inches (5 cms) in length. 

They have a diamond-shaped with a shiny reddish, pinkish, orange-bronze colored body. The shade varies depending on the genetics of the parents. The conditions of the water in which they live influence hue greatly.

They have a black wedge pattern on their body, starting approximately from the midsection of dorsal fins till the end of the caudal peduncle. The black pattern follows the body the shape of the fish hence making it a triangular shape. The abdomen’s color is white.

With all the Cyprinids, the pectoral and pelvic fins follow a certain pattern. The pectoral fins are located behind the operculum and the pelvic fins are located along the ventral portion of the body.

All the fins, dorsal, pelvic, anal, and caudal fins are translucent with a hue of pinkish-bronzy, its body color. 

The male of the species is slender, while females are more rounder and bigger. The black wedges are a little larger and rounder at the caudal peduncle section.

The variations of harlequin rasboras, such as Black Harlequin Rasbora, Blue Harlequin Rasbora, Purple Harlequin Rasbora, and Gold Harlequin Rasbora depending on the shades of the fish.



Harlequin Rasbora Life Span:

The average life span of Harlequin rasbora ranges from 5 to 8 years, depending on the optimal condition of the water.

The factors that influence the life expectancy of Harlequin rasbora are the quality of water they live in, the tank mates, and the genetics of the parents.

Rasbora Harlequin Habitat:

Rasbora Harlequin is a member of the Cyprinidae family in the order of Cypriniformes. They are native to the streams and water bodies from Thailand, Malaysia, and Singapore.  

They inhabit the streams and water bodies which is of high humic acid concentration and low mineral contents. This condition is the consequence of the waters flowing through tropical forests where waterlogged soil prevents the putrification of dead leaves and other green matters completely.

Consequently, it infiltrates humic acids, fulvic acids, and other compounds specific to humification into the water that flows through these forests. 

Therefore, the water of the streams where Rasbora Harlequin live ranges from a pH of 5 to 7 with a dGH range from 5 to 12. The general water temperature in their natural range from 71°C – 77°C (22°C – 25°C).

Their natural habitat is covered with dense vegetation with dark rocky and sand substrates. 

Behavior and the temperament:

Trigonostigma heteromorpha is a peaceful species and makes an ideal member of a well-maintained community tank.

It is a schooling species by nature and should keep them in a group of at least 8 to 10 specimens. 

They are more or less stay at the middle level of the tank, rarely venturing to the top or bottom of the tank. As a group, they are active and vibrant, and alone, they are shy and hide amongst the plants and the other decors in the tank. 

They do not nip or bother other species in the tank. Often, some boisterous species in the community tank can trouble them and stress them out. Hence, it is important to choose carefully the inhabitants of the habitat where harlequin rasbora inhabits.

Tank Size and Conditions for China Danio:

Tank Size: 

You need a minimum tank size of 10 gallons (40 liters) to keep the harlequin rasbora. As they thrive better in a fairly big group, we recommend you keep a group of 8 to 10 fish at least. If you are planning on keeping more fishes, consider buying a bigger tank. Despite their nano size, they need plenty of space to swim.

Besides, you need space to plant and other decors to recreate their original habitat of rasbora harlequin.

Tank Conditions and water parameters:

Harlequin Rasboras are native to the streams of Thailand, Malaysia, and Singapore.

In their natural habitat, rasbora harlequin inhabits slow-moving streams of water that flow through tropical forests where waterlogged soil prevents the putrification of dead leaves and other green matters completely. 

Consequently, it concentrates the water with high humic acid and related substances and low mineral contents. Therefore, their natural habitat stays slight on the acidic side pH of 5 to 7 with a water hardness 5 to 12 dGH.

The stream water temperature range from 71°C – 77°C (22°C – 25°C) 

However, you can keep them the tank where the temperature range from 70 to 82°F (21 to 28 °C).

The best way to maintain a stable temperature range is to get a high-quality heater that maintains the set temperature effectively.

Although harlequin rasbora is a fairly hardy species and adapts to the local aquarium conditions they thrive their best in water conditions closer to their natural habitat.

While creating a habitat for Trigonostigma heteromorpha, you should try to replicate as close as possible their natural habitat if you want them to grow to their fullest potential and to live to their maximum lifespan.

Filtration and other equipment:

There is no specific requirement for filtration. These species inhabit slow-moving streams, which means you will have to take care of the filtration and aeration systems accordingly. You can go with a simple sponge filter media or hang-on-back filter or a canister filter. 

If you use a hang-on-back filter or a canister filter sure to keep filtration inlets and outlets in the tank covered with a sponge pre-filter.

You need a heater if the temperature dips below 70°F (21°C). You may need some small supplies such as a thermometer to keep watch of the temperature of the tank water, a gravel vacuum for a weekly water change, and a test kit to check pH and the other water parameters.


Rasbora Harlequin predominantly dwell at the middle level of the tank, they rarely venture up to the surface of the tank or the bottom of the tank. Therefore, they can thrive in most substrates such as rocky, gravel, plant. Just avoid too fine sand substrate for any nano species.

You can choose the substrate according to the need of the other tank mates and the plants. My recommendation is to choose a dark substrate as their natural habitat is with subdued lighting. Also, it is aesthetically appealing as the dark substrate brings out their best colors. 

Lighting and Decors:

Though they are not timid fish, they like to have plenty of hiding places due to their size. Having plenty of hiding places in the tank makes them feel safe and secure.  

Good decor for harlequin rasbora is to plant plants that are tall and reach to the surface of the tank and some broadleaf plants such as Anubias nana with some open space to swim. You can also add some hardy floating plants such as java moss which provides both the extra shelter and the micronutrients for the small fish.

Bogwood is a great addition as it lowers the pH and hardness of the water by absorbing peat tannins, which helps to maintain the water parameters closer to their natural habitat.

However, do not go overboard with your plants. But, do it with moderation. It should be just enough for the fishes to hide, rest, and play. Leave enough room for them to swim around, as they are one hell of an active species.

Harlequin Rasbora Tank Mates:

Harlequin Rasbora is a shoaling fish, you need to keep them in a group of 8 to 10 at the least. Keeping a lone individual harlequin in a community tank will not serve any purpose. Alone they are shy and get stressed. 

Besides, the beauty of the harlequin rasbora lies in schooling. The shape and the black pattern on them are not distinct when alone, only a group of schooling harlequin rasbora can create a dazzling effect on the spectator.

While choosing tank mates for harlequin, make sure not to overlook their need for the environment in which they thrive their best. The water parameters, plants, decors, and open space for harlequin to swim around.

Adding big or boisterous or aggressive fish is not an option as a tank mate for rasbora harlequin. 

You can choose other peaceful schooling community fish such as Danios, Tetras, such as neon tetra, ember tetra, cardinal tetra, platies, mollies, dwarf gouramis, small barbs, corydoras, and other rasboras.

You can also have invertebrates like loaches, kuhli loach, zebra loach, red cherry shrimps, Amano shrimps, ghost shrimps, etc.

Can Bettas live with Harlequin Rasbora?

Housing Harlequin Rasbora with Betta is quite popular among aquarium hobbyists. The reasons for housing them together are:

  • They both are native to Southeast Asia.
  • They both live in slow-moving water streams that have the same water parameters.
  • They both need the same kinds of food and living conditions.
  • Harlequin Rasbora is a fast swimmer while betta moves slowly displaying its fins and tails.
  •  Last but not least is they both together in a tank exhibit a spectacle that is a feast to the onlookers.

The chances of betta attacking harlequins are slim as they are fast swimmers. 

However, some harlequin rasboras may try to nip bettas fins. In that case, you may have to remove the individual harlequin which is causing the trouble. Alternatively, you can move the betta to another tank.

Harlequin Fish Diet:

Harlequin Rasboras are omnivores. In the wild, they eat a wide range of food such as insects, larvae, worms, zooplanktons, planktons, etc.

You can feed them with high-quality commercial flakes and pellets, some frozen/live brine shrimps, bloodworms, daphnia. Also, feed them with some blanched vegs such as spinach, cucumber, zucchini, lettuce, etc. Feed them small meals a few times a day than one or two large meals. 

Feed them the quantity that they can finish within a few minutes. Remove leftovers from the tank after half an hour to 1 hour to prevent the decaying of the food.

Decaying of the food produce ammonia, nitrite, and nitrate faster than what the biomass in the filter cartridge can handle.  

The biomass contains many beneficial bacterias/enzymes, that will digest the ammonia and convert it into a byproduct called nitrite (a toxic byproduct), and then to nitrate. 

This process is called the nitrogen cycle. Nitrate is a lesser-known toxic amongst ammonia, nitrite. Nitrate is colorless and odorless; the only way to detect is to measure with ion-specified electrodes (ISE). 

Algae bloom usually is an indication of high nitrate levels; algae start to grow with a nitrate level of 10 ppm. The maximum limit for nitrates in the freshwater aquarium is 50 ppm. According to Tropical Fish Magazine, most fish don’t show any sign of nitrate poisoning until it reaches the level of 100 ppm.

Algae bloom can lead to drowning or suffocation of your fish to death. 

Harlequin Rabora Care:

Harlequin Rasboras are hardy species, and the disease is not an issue in a well-maintained tank.

The common diseases affecting them are fin rot, dropsy, and Ich or white spot disease if their habitat is not well-maintained. The outbreak of the disease can be one or more fish, the best is to deal with it at an early stage.

Maintaining the right water parameters with balanced nourishments you can avoid both dropsy and ich, and to some extent fin rot as well. 

Skin rot is a common disease faced by aquarists due to poor quality of water circulation, wounded fins from fighting or fin nipping or scratching on the sharp edges of decors, etc. 

You can minimize or completely avoid by maintaining the recommended water parameters, running the filters constantly, and cleaning the tank periodically. Avoid decors with sharp edges.

You can isolate the individual fin nipper to prevent the fins from getting wounded.

Breeding Harlequin Rasbora:

Harlequin Rasbora is a bit tricky species to breed in an aquarium tank, but it is not impossible. 

Water parameters should match their original habitat. The temperature of the water should be in a range of 78°F – 82°F ( 26°C – 28°C), with a pH range 5.0 – 7.0, dGH range 5 – 12 for them to willing to spawn. 

For the breeding tank, you can use peat filtration to condition the water to mimic their natural habitat. You can also use a sponge filtration system. Keep the water movement to a minimum. 

Plant broad leaves aqua plants such as Cryptocoryne, Anubias nana, Limnophila, etc. you can also have some java moss to harbor infusorian. Infusorian is the first best food for the fry.

Unlike other rasboras, which are egg scatters, Harlequin Rasboras are egg layers by attaching the eggs to the underside of the broad leaves or similar surfaces.

Choose bright-colored specimens and start conditioning them with good quality live foods such as daphnia, brine shrimp, blood worms, mosquito larvae, etc. and introduce the breeding pairs into the prepared tank.

The best time to move them to the breeding tank is in the late afternoon or the evening. Generally, Rasbora Harlequin spawns in the early mornings. 

When ready to spawn the male will guide the female to a chosen underside of a leaf. While spawning females will invert her position rubbing against the leave and the male will approach the female and take the same inverted position against her. 

As the females stick the eggs to the underside of the leaf, the male will spray the milt and fertilize the eggs. Anywhere between 6 to 12 eggs will be deposited at a time by the spawning pairs. They repeat this course of action for approximately 2 hours depending on the condition of the female. A well-conditioned female will lay over 100 eggs.

Once they spawn move the breeding pair to their original tank as they consume the fry. The fry will hatch in about 18 to 24 hours at the right temperature 82°F(28°C).

Harlequin Rasbora spawning

For the first couple of weeks, the fry feeds on infusorian which is abundantly present in well-grown plant-like java moss or else you can feed them with infusorian water. 

After two weeks you can feed them with young brine shrimp, and commercially prepared fry food. It takes about 8 to 10 weeks for the fry to take adult form.

Final Thoughts – Harlequin Rasbora:

Harlequin Rasbora is one of the most popular among the Rasboras. They are peaceful and hardy fish at the same time a school of Rasbora Harlequin will dazzle you with its beauty.

It is a perfect beginner’s fish. In a well maintain habitat they live for over 6 years. They are easily available both online and pet stores and are quite inexpensive fish.

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