Harlequin Rasbora Stats InfographicHarlequin rasbora, also known as trigonostigma heteromorpha or rasbora heteromorpha, are peaceful shoaling fishes.

They have a beautiful shimmery color which creates such a fascinating view through a glass aquarium. However, they are quite small-sized, making them prey to bigger tank mates.

Read about these middle strata species and learn how to keep your harlequin rasbora alive and healthy.

Harlequin Rasbora Stats

Species: Trigonostigma heteromorpha
Common Name: Harlequin Rasbora
Size: Up to 1½ inches (4cm)
Habitat: ASIA; Forest streams and rivers in Thailand, Malaysia, and Sumatra.
Min Tank Size: 15 gallons for a school.
Diet: Omnivorous, flake, Frozen, and live food.
Behavior: Peaceful, lively. Keep in schools of at least six.
Water: Temperature 72 to 77 F (22 – 25 C) pH range: 5.0 – 7.0; dH range: 5 – 12
Care: Easy, needs open swimming space.
Communities: Excellent
Suitability: Good


Listed tank sizes are the minimum
Size: 1 3/4 inches (5cm)
Tank: 20 inches
Strata: Top, middle
PH: 5 to 7
Hardness: Soft to medium
Temperature: 73 to 82 F (22 to 25 C)


Order: Cypriniformes
Suborder: Cyprindei
Family: Cyprindae
Genera: Trigonostigma

– Name

Harlequin Rasbora, Red Rasbora, trigonostigma heteromorpha or rasbora heteromorpha and Harlequin

– Origin And Distribution

The harlequin rasbora is originally from some parts of Asia. They are widespread in the swamp forest, rivers, and streams of Singapore, Eastern Sumatra, Malaysia, and Thailand.

Their natural habitats look similar to the fresh black waters of South America. The water gets its color from decayed leaves and waterlogged soils. The swampy vegetation areas they inhabit comprise minute amounts of minerals and an increased concentrates of humid acid.

– General Body Form

Harlequin rasboraThe harlequin rasbora is distinct from the other rasboras species. It has a convex-shaped stomach with a compacted build that narrows down at its mouth and caudal fin areas. The harlequin rasboras have a diamond shape with a tapered tail. Female harlequin rasbora is bigger with a more dominant appearance than the male species.

The harlequin rasbora size is minute; they grow to about 4.5cm in length while in captivity. Their body seems tall or stretched, particularly around the midsection region. While the males look rounder at their anal fin regions with more prominent body markings.

– Coloration

The harlequin rasboras get its name from its striking black triangle patch that resembles the popular harlequin attire. The mark looks wider at the beginning but narrows at the end near the caudal fin region. The female harlequin rasboras pattern does not look defined at the beginning of the triangle, but it is more apparent in the males as it further extends to their stomachs. The harlequin rasboras marking resembles the pattern on the black harlequin rasbora.

However, the remaining portions of the harlequin rasboras have no pattern giving it a strong contrast. The background color of the harlequin fish is silvery and shimmery with random orange hues. Its fins are deeper orange color but, it differs in intensity amongst different fishes. The color variation relies on the tank conditions, the comfort levels of the harlequin rasboras, and their parents.

The anal and pelvic fin is translucent. Their dorsal fins are red with a yellow edge, while both the up and down caudal fins are a brighter red shade with faded yellow color. The sides of the harlequin rasboras are a wide range of pinkish hues to a reddish-copper shade.

– Behavior

The harlequin rasbora are peace-loving and would not threaten other fishes around them. They are middle-level species that spend their time mainly there, and they hid away in caves or tank shelters when other fishes are chasing them or when the aquarium lights are too bright. These fish enjoy being in a school and would exhibit shoal behaviors with a hierarchy system.

A juvenile harlequin rasbora is shy and likes to be part of a big school, but as they mature, they try to climb the school’s hierarchy to attract a mate. The males dance with their school members by going around in circles two centimeters apart. As they swim around, they begin to display wider fins that occasionally tremble simultaneously. The winner attains an honorary position in the shoal while the weaker fish accepts defeat.

However, some competitive females get involved, but it is a rare feat. They dance or fight, usually not violent, and none of the harlequin rasboras gets injured even when their counterparts make sudden movements. They are happiest amongst similar fishes but sad alone in a tank. In fact, the larger the fishes’ schools are, the more they glow.

– Lifespan

The harlequin rasboras live between 5-8 years but an average of six years. Their lifespan is influenced by exceptional care, feeding, and of course, genetics. A healthy and happy harlequin rasbora would live longer than a depressed one living under poor conditions.

Harlequin Rasbora Care Guide

A harlequin rasbora care is minimal with few demands. In its natural habitat, the harlequin rasbora stays in fresh tropical blackwater swamps. You have to mimic its home in your tank, so we have put together some features.

1. Water Parameters

You would need a heater to regulate the harlequin rasbora temperature; they need to be warm as they are tropical water fishes. Their tank water temperature should remain within a range of (73-82) F or (22-25) C and a neutral PH range of 5 to 7. It is crucial to note that alkaline or acidic water is inadequate for your harlequin rasbora. They prefer slow-moving waters and are quite tolerant towards slight water parameter differences.

Maintain a water hardness range of 2 to 15 KH, and you can monitor it by running frequent tests. You can get a portable yet accurate test kit from fish stores near you. They are naturally from soft water habitats, but they have been tank-bred for a long time, so the aquarium water’s chemical composition does not negatively impact the harlequin rasbora. However, you should pay attention to every single water parameter detail during breeding as they would significantly differ.

2. Substrate

The substrates should be dark like a waterlogged soil substrate with many aquarium plants, rocks, and gravel. The dark color of the substrate makes the harlequin rasboras appear less stressed. However, because they are mainly middle dwellers, the smoothness of gravel or rock is not crucial as they are less likely to get injuries.

3. Aquarium plants

Grow some aquarium plants like Cryptocoryne species and put them around the aquarium to help them feel comfortable in a community tank. The Cryptocoryne plants are an ideal suggestion because they are popular in their wild habitats, and they are not too sturdy for the harlequin rasboras to stay in between or swim through. Water lettuce, Cryptocoryne wendtii var, Cryptocoryne usteriana, or Cryptocoryne lutea can remain at different distances apart in your harlequin rasbora aquarium.

They would occasionally hide in between the vegetation but be sure to leave space for swimming activities. The aquarium plants would provide oxygen and cause an aeration effect which means you do not need artificial equipment.

4. Tank Decor

Most tank decor you would put into the harlequin rasboras aquarium, like rocks or pebbles, are not crucial to the harlequin rasbora.

5. Filtration

You do not have to install a special filter as a standard hang-back variant would work for the harlequin rasbora. You should only put in your school (of at least ten harlequin rasbora) when you have completed the nitrogen cycle. It is because they are likely to get ammonia poisoning and die if the tank parameters are unsuitable. You have to perform regular water filtrations to keep them safe.

6. Harlequin Rasbora Tank Lights

The harlequin rasbora requires minimal lights. You can use the aquarium plants to block off some of the light beams if the lights are too bright.

7. Harlequin Rasbora Tank Size

The harlequin rasbora tank should be a minimum of 20 inches or a 10-gallon aquarium. They are indeed small-sized, but they need ample space to shoal. On average, two harlequin rasboras require a gallon of water and space: the larger your aquarium, the more room for their fascinating shoaling presentations.

Tank Mates

Harlequin rasbora tank matesChoosing harlequin rasboras tank mates is easy when you follow basic rules. You should consider the size of both species; the harlequin rasboras and the prospective aquarium mate. A harlequin rasbora is a very calm fish that would hardly interfere with other fishes.

It is crucial to their lives that you do not put them with big fishes, predators with giant mouths: it is because they would harass the harlequin rasbora and, in most cases, try to eat them.

Examples of fishes to avoid are bettas, clown loaches, large cichlids, Opaline gourami, black neon tetra, and cardinal tetra.

  • Bettas may not be ideal tank mates, but they can live together because of the peaceful nature of the harlequin rasboras. However, the bettas can be too boisterous for them and harass them.

Suitable tankmates for the harlequin rasboras are; Mollies, Tetras, Kuhli Loach, Bolivian Ram, small-sized Rasboras, Rummy Nose Tetra, Apistogramma, honey gourami, cherry barbs, Guppies, zebra coaches, dwarf gouramis, platies, hatchet fish, Danios, and corydoras catfish.

  • Co-specific is the best choice because they are not aggressive or territorial species. The harlequin rasbora enjoys each other’s company and gets along so well. They spend time exploring together but would go off to be alone once in a while.

Snails and shrimps are also a good suggestion because they are peaceful and do not disturb the harlequin rasbora in any way.


The rasbora species spawn quickly with minimal requirements and triggers, but the harlequin rasbora breeding is different. They are so particular about everything, especially water and tank parameters.

– Tank and Water Parameter

The harlequin rasbora breed in soft water; you should maintain a hardness range of 1.5 to 2.5 dH. Also, the water PH must be neutral, like 6.0. They prefer a spacious tank, and mimicking their natural habitat is a good idea; put in dark substrates and minimal lightning. To simulate the high humic acid concentration, put some peat moss in the tank water. However, it is optional and not a crucial breeding factor.

– Mating Process

Select healthy mates and feed them nutritious meals like mosquito larvae, blood worms, and daphnia before they breed. The harlequin rasboras are oviparous; they lay eggs. The female harlequin rasbora would lay eggs while the male harlequin rasbora fertilizes them. Multiple breeding sessions can occur in one tank simultaneously, and make sure the female is more than the male with a ratio of 2:1.

Increase the water temperature to trigger the female harlequin rasbora to spawn. The male would perform a seductive dance to entice the female. The female is also active during mating; She encourages the male to float to the aquarium top. Then both entangle themselves while facing their abdomen region upwards.

The actual spawning takes time as they like to be comfortable. She needs a spawning surface like an aquarium plant with broad leaves (like the seedbox, Anubias Nana, water hawthorn, or Cryptocoryne). When the pregnant female is ready, she uses her stomach to rub the spawning surface. It serves as a signal to the male. The male and female would lay upside down beneath a leaf, or the male would curl his tail to trap the pregnant harlequin rasbora.

The eggs would float and stick to the leaf as the female lays them in batches. They may be a total of 300 eggs by the end of the spawning process.

– After Breeding

The male and female harlequin rasbora does not pose any sentimental attachment or instincts to protect their eggs. They tend to eat them if you do not take them out from the tank. To ensure the harlequin rasbora eggs hatch, leave them on the spawning surface to incubate for a day or 30 hours. Set the water temperature to about 28 C or 85 F and monitor, or so it does not change suddenly.

The fry absorbs its highly nutritious yolk during the incubation period. Reduce the water in the breeding tank to about four to five inches so the juvenile can seem to them too quickly. They have under-developed lungs and can drown if the depth of the water is too much. The youngster begins to swim at most after five days and grows fast.

You should feed them cyclops, nauplii baby brine shrimps, and fine infusoria for two weeks. Then introduce them to small-sized fish food. An inadequate diet can delay its development and growth as a harlequin rasboras mature within six months.


The harlequin rasbora diet comprises small portions of food because it is tiny and has a small mouth size. They are omnivorous and would eat both fleshy and vegetable meals. These fish eat well and are usually boisterous during this time. They would eat anything but feed them only safe meals. You should add food suggestions like eggs, plants, insects, larvae (mosquitoes), daphnia, blood worms, and shrimps.

Buy quality fish foods from a fish store and pay attention to the feed’s size, so it easily fits into their mouth. Switch up the meals to not get tired of eating the small thing every time; they find it boring. You can add tiny-sized pellets and flaked fish food to their diet. Offer your harlequin rasboras special treats once or twice weekly to drive them something to look forward to; it excites them.

If you have the time, make the fish food yourself and add in some vegetables. You should feed your harlequin rasboras two times a day and make sure they eat enough but do not overfeed them. You can utilize the one-minute feeding technique. Feed them continuously for a minute and allow them to eat and stop the time immediately is up.

As a rule, take out leftover food from the tank to prevent decay and ammonia poisoning. Occasionally, mix up the harlequin rasboras food by alternating dry meals with live food.

Common Diseases

Harlequin rasboras are resilient and would hardly fall sick, and that’s why there are not a lot of likely illnesses they contact. However, as freshwater dishes, they are susceptible to a few freshwater parasites, particularly when a new fish joins the aquarium without quarantine. The possible harlequin rasboras diseases are ich. It is due to stressful tank conditions and dirty water.

Other diseases that may affect the harlequin rasboras are likely to occur due to inappropriate living conditions. To reduce the probability of your harlequin rasboras falling ill, perform regular water filtrations and regulate its water conditions. Use your test kits to check the PH and water hardness frequently.


  • Harlequin rasbora known as trigonostigma heteromorphaThe harlequin rasboras are tiny peaceful fishes.
  • They eat small portions of vegetable and little-sized pellets because of their mouth size.
  • These fish breed quickly when their tank and water conditions are perfect.
  • They are shoaling fishes.
  • Their ideal tank mates are non-aggressive similar-sized fishes.
  • They are not susceptible to diseases but may contact a parasite under poor conditions.

The harlequin rasboras are attractive fishes that require minimal care that even beginner aquarium effortlessly perform. You should get a shoal of these shimmering harlequin rasboras to enjoy their breath-taking swimming displays.

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