Harlequin Rasboras (Trigonostigma Heteromorpha)

harlequin rasboras

Last updated on August 1st, 2022 at 08:10 pm

Harlequin rasboras is a genus of small colorful territorial fish belonging to the family Cyprinidae. They are distributed in rivers and streams of Thailand and their surrounding regions.

The name Harlequin Rasbora is derived from their patterning which is reminiscent of a harlequin costume, hence the name of the genus. The fish are active swimmers who live in acidic riparian habitats. They are nocturnal and are an important food source for ducks, lizards, birds, and some other big fishes.

Origin and description

The fish is native to Indonesia, where it is known as the Harlequin Rasbora. There are two types of these fish, one with grape-red streaks, and the other with a blue-green striping on the tail. They normally grow to a size of two to four inches but may grow larger if given enough space to live.

They like to live in acidic water but can live in more neutral water, provided the water is not too soft. The males and females are about the same size and grow to about 4 inches. The male is a bit larger, and has a wider striping, while the female is more sparse. They prefer to live in a school.

The Harlequin rasboras were originally described by John Henry Seebohm in 1887.

Species profile

harlequin rasboras

Females develop white body pigmentation to blend in with their surrounding environment. Harlequin rasboras are famous for their clashing pattern which becomes more apparent with age. These delicate little fish are often low-maintenance and make charming pet fish.

They are inquisitive and require little time and effort to care for. This profile of the harlequin rasboras describes the stunningly beautiful fish, but also the drawbacks it may have to provide care to.

Scientific name

The scientific name of Harlequin rasboras is Trigonostigma heteromorpha (Lat. meaning “triangle mouths”) and the fish is a genus of freshwater fish of the small rasbora genus which has been split off from the main rasbora because of the greatly enlarged eyes.

Habitat and distribution

These tiny, orinine (from the Latin word orinus meaning “river”) rasboras, native to South and Southeast Asia are available in many colors, including white, black, and orange, that accommodate any aquarium.

They are generally peaceful and can be combined with other species. They are suitable for a 10-15 gallon long aquarium.

Trigonostigma heteromorpha are extremely peaceful and hardy fishes once acclimated, and are peaceful with other fishes. The male is also known to grow as long as 5 inches or more and the females are considerably smaller, but nevertheless make up for it with their friendliness.

They are vivacious males and they will scamper across the top of a tank, displaying to the females. For this reason, it is extremely important to introduce this species with a rival male in the tank so that it will display. It is also recommended to keep them in groups because they are becoming more elusive in nature, as most are found in small shoals.

Harlequin rasbora size

The harlequin rasboras is a very small fish, with males reaching around 10 cm (4 in) in length, and females being about 8. 4 cm (3 in) long.

The tank size for Harlequin rasboras should be about 30 gallons for two fish.

Harlequin rasboras tank size

Harlequin rasboras are recommended to be kept in a tank of no more than 50 liters (13 gals). This fish is territorial, so it must be kept in a group of no less than 5 individuals. It should be kept with other smaller shoaling species, especially similar species, but it should be combined with similarly sized fishes that breed together. Males are extremely aggressive, even virgin males may eat the eggs of a group’s females.

Harlequin rasboras tank size

Harlequin rasboras need the same tank décor as other Rasboras, including numerous hiding places, plants, and driftwood. A well-planted fish room will provide the ideal spot for them to live. A spacious aquarium that is less than four times the length of the rasboras root is a good place to begin.

Plants should be added as they become available, and the harlequin rasboras should not be crowded. This will give the fish room to swim without feeling threatened. A small group of harlequins can hang out on the planted surface, or they can live along the base of the tank.

Keep at least four inches of water above the base of the roots.

What fish go well with rasbora (tankmates)

Harlequin rasboras can be kept with any fish, as long as the fish is not large and predatory.

Some good tankmates are Tetras (Like Neon tetra, Cardinal tetra, and Black Skirt), Shrimp, Cory Catfish, Guppies, Mollies, Snails, Platies, Danios, Plecos, Hatchetfish, Dwarf Gourami, Zebra Loaches, and other small rasboras.

Harlequin rasbora breeding

harlequin rasboras

In order to breed harlequin rasboras, first set up a separate breeding tank containing broad-leafed cryptocoryne plants, the rasbora’s favorite place for laying eggs in its natural habitat.

After that, place an adult male well-conditioned with a female, heavy with roe in the breeding tank late in the day. Initiated by the male and including the usual fin-flaring and dancing, courtship begins early the next morning. After swimming together for a while, the two move under a broad leaf of a plant. The female turns upside down and lays a few eggs. They move off to court again before returning to spawn again, which might take place on the same plant or leaf.

25 to 100 eggs will be deposited by a healthy pair and later fertilized by the male. Occasionally, Rasboras will eat eggs, but they are not avid egg eaters. So after breeding, it’s a good idea to remove the parents.

It takes about 24 hours for the fry to hatch and they are free-swimming on day three. For the first week, feed them infusoria or liquid fry food or egg yolk squeezed through a cheesecloth. You can then gradually wean them onto baby brine shrimp and commercial dry ground fry food.

When well-fed and regularly, harlequin rasboras grow extremely fast and reach adulthood within three months. The sponge filter should be added after the first week, and water should be changed every week after the first month.

Are harlequin rasboras aggressive?

No, they are peaceful. Their peaceful nature means they get along well with other peaceful species, but they can catch the attention of larger species.

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Harlequin rasboras care

harlequin rasboras

The Harlequin Rasboras is widely recognized as a staple fish to have in a community tank. They are able to withstand most water conditions, so they can adapt to almost any environment. They are peaceful and make good community fish. They require a tank size of at least 10 gallons. If the water temperature is below 70 degrees F, they will mostly be a substrate feeder.

What they eat

Rasboras are omnivorous and enjoy a varied diet of high-quality flakes or granules, as well as live foods such as daphnia and artemia. In addition to this diet, you can provide your pet with meaty options such as freeze-dried bloodworms or tubifex worms. Rasbora also welcomes fresh vegetable treats on a regular basis.

Water conditions

Rasboras, especially members of the genus Boraras, prefer a pH of 5.0 to 7.0, KH between 1°and 3°, and a temperature between 75° and 80°. They need also need a good filtration system.

The infected fish needs aquarium salt, chlorine-free tank water, increased water temperature, and over-the-counter medication like API Fungus Cure.

External parasites on rasboras’ bodies can cause white spots to appear on their skin and gills. This disease can be treated by raising the water temperature or using over-the-counter medication.

This is basically kidney failure. It is caused by excess ammonia and nitrites, and can cause swelling in the abdomen.


The Harlequin Rasbora has an average life span of 5 to 8 years, depending on the condition of the water.

Pests and diseases

harlequin rasboras

Rasboras are hardy fish found in streams, rivers, and swamps, which is why they don’t get sick easily. Listed below are some diseases your beloved fish may suffer from.


Freshwater fish are only affected by this disease, which is highly contagious.

Causes: If oxygen levels in the tank are low, and the temperature is 80 Fahrenheit.

Signs: Ulcerations on the skin, accumulation of mucus in the gills and dorsal fin, noticeably long periods without eating.

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During the treatment, stabilize the water temperature and consult the vet for antibiotics or use API Furan-2 as an antibiotic.


It is basically kidney failure. As a result of water accumulation in the body, the abdomen swells.

Aeromonas bacteria and water with excessive ammonia and nitrites are the causes.

A bloated belly, stringy feces, loss of color on the gills, and whitening of the scales are all signs of this disease.

Fin and Tail Rotting

Eventually, the tail and fin tissues start rotting outward. It’s possible that your fish’s rotting might be due to wounds already on it, or it could be because of unclean water.

There are several causes of this condition: dirty water, overfeeding, poor nutrition, and stress.

Inflammation of the fins is the first sign. When the fins lose their color, they become brittle and weak.

Cleaning the water, making sure it is at the right temperature and pH, and optimizing the pH is the best treatment.

Set the temperature to 77 degrees Fahrenheit, and use an over-the-counter medicine like API Fungus Cure.

Ich/white spot disease

Rasboras are also usually infected with ich/white spot disease, another widespread disease.

White spots will appear on the fish’s skin and gills when external parasites attach to its body. Causes are Ichthyophthirius Multifiliis (an Ectoparasite), contact with other infected fish, sudden temperature changes in the aquarium, and decorative plants that contain parasitic cysts.

Symptoms include: Crystals forming on the body, slimy skin, asthmatic breathing, itching against rough surfaces, and hiding for days.

Other symptoms include patches of skin and gills, a slimy layer on the surface, and browning of the skin. If a fish is infected, it must be isolated from its tank mates, aquarium salt should be added to the tank water, chlorine should be removed from the water, and the aquarium temperature should be raised.

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You can use an over-the-counter medicine like Ich-X or bring the water temperature up to 71.8 to 86 degrees Fahrenheit as a treatment.