Hot Water Fish

Harlequin fish

  • Scientific name: Trigonostigma Heteromorpha
  • Common Name: Harlequin Fish, Rasbora heteromorpha
  • Aquarium size : 100 liters
  • Temperament: Calm
  • Temperature :  Between 21º and 28ºC
  • pH: Between 5 and 7.5
  • Diet: Omnivores
  • Length: 4.5 cms


The Harlequin fish or Harlequin Rasbora, is a fresh and warm water fish belonging to the Cyprinidae family .
It has been present in aquariums since the early 1900s, when the first Harlequin fish arrived in aquariums.
Its growing popularity is due in large part to being a highly disease resistant fish , in addition to being a very sociable fish, an attitude that allows it to share space with a great variety of different species, we could say that it is one of the 10 fish suitable for beginner aquarists .
At present we can find different varieties of the harlequin fish: Rasbora Harlequin (golden color), Black Rasbora Harlequin and Blue Rasbora Harlequin.
Different studies have confirmed that the species of Harlequin fish available for sale contain morphological incongruities, the result of crossing between different varieties.


The Harlequin fish is a really small fish, in freedom it does not usually exceed 3.2 centimeters, although in captivity it can grow somewhat more, up to 4.5 centimeters.
Its usual color varies from reddish, pink or orange, with a black stripe in the shape of a triangle, which runs from the dorsal fin to the middle of the base of the tail fin.
The colors can be altered under different circumstances, such as the quality of the water, the health of the fish, even depending on its mood and the mating season. They have an oval body, compressed on the sides. The Harlequin fish’s mouth is oriented upwards, possibly because it is a fish that feeds on the surface of the water.
Between males and females, there are no big differences,Except that adult females are usually rounder and larger than males, thinner and more colorful. Another way to distinguish females from males is that the triangular mark is sharper and better defined in males, while in females it has a more rounded outline.

Distribution and habitat

This species of fish is native to southern Thailand, Malaysia, Singapore, and the islands of Borneo and Sumatra.
It seems that the species caught in southern Thailand are somewhat smaller and thinner than those from Singapore and the islands of Borneo and Sumatra.
Its natural habitat is gentle currents, with an abundance of vegetation. The water is usually soft, weakly acidic and with little light, due to the large amount of vegetation present, since these waters usually belong to rivers with a large number of trees.

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Aquarium conditions

The size of the aquarium should be about 100 liters, with a dark substrate bottom or in its place and recreating its natural habitat, a sandy bottom should be placed in which to include roots and wooden branches, which form shady holes and caves , where they can hide.
In addition, dry beech or oak leaves can be added to the aquarium, to give a greater feeling of naturalness. These leaves, when they begin to decompose, will serve as food for the fry.
Aquarium lighting should be dim. The waters in which they move are not very illuminated and a change of environment would not suit them.
Its ideal temperature for water ranges between 21º and 28ºC, with a pH between 5 and 7.5.


It is an omnivorous fish , which in the wild feeds on small insects that fall into the water, worms, crustaceans and zooplankton. In the aquarium, to maintain a balanced diet, we must provide good quality dry food, supplemented with live foods that we can get ourselves, or, failing that, frozen foods, such as earthworms, Daphnia and Artemia.

Behavior, Harlequin Compatible Fish

Compatible fish without restrictions

  • Botia Clown
  • Guppy endler
  • Guppy
  • Scalar Fish
  • Otocinclus
  • Gourami Pearl
  • Telescope
  • Tiger Barbel
  • Ax fish
  • Nun Fish

Compatible fish without restrictions

  • Tetra Borrachito
  • Snail Apple
  • Comet Goldfish
  • Molly fish
  • Ramirezi
  • Corydora
  • Neon Tetra
  • Female betta
  • Mosquito fish
  • Barbel Cherry

Compatible fish without restrictions

  • Ancistrus
  • Platy Fish
  • Chinese Neon
  • Rasbora Galaxy
  • Silver Dollar Fish
  • Danio rerio

Restricted Compatible Fish

  • Rainbow Fish 
  • Discus fish
  • Fish of Paradise
  • Killi fish

We are facing a very calm species, which does not have problems with any type of fish, except with those that by size can eat them.
They are gregarious, so it is advisable to maintain a small community of Harlequin fish. Experts recommend having between 8 and 10 individuals, so that we will achieve a more natural effect, that the males show their best colors, and mainly, we will ensure that they are calmer, with better health.
In a community aquarium, they could share space with other cyprinids, tetras, small cichlids , even some varieties of Betta .


Harlequin fish can be raised in an aquarium , provided the conditions are right.
The first thing we must know is that this species does not show special care for its eggs, nor for its fry.

  • They reproduce easily in a well-established aquarium (fish that are at least one year old), and in which there is adequate vegetation, since they tend to attach their eggs to the underside of broad leaves.
  • For greater control of the Harlequin fish reproduction process, a breeding aquarium can be installed.
  • The aquarium must be conditioned with weak lighting, with a mesh-like base or similar, so that the eggs that do not stick to the plants and fall to the bottom, are not within the reach of the adult fish, which could eat them.
  • The water temperature must be extremely high in the ideal situation, that is, over 28ºC, with a pH between 5 and 6.5.
  • The weeks before spawning, they should be fed 2 to 3 times a day with small amounts of food, preferably of animal origin: insects, small crustaceans …
  • We will have evidence that spawning is close, when the males show more vibrant colors and the females appear more voluminous.
  • Spawning usually occurs in the early hours of the morning , and is preceded by a great courtship activity of the male, with races throughout the aquarium.
  • The female will begin spawning in small groups, which are fertilized by the male. It will always put a new group, when the previous one is fertilized.
  • Once the spawning has finished, it is advisable to remove the adult fish, so that they do not eat the eggs.
  • The eggs hatch between 24 and 48 hours later. The fry will swim perfectly, a week.


The Harlequin fish is one of the suitable fish for beginner aquarists , due to their ease of handling, compatibility and durability, they can live up to five years.
For better health, the ideal is to try to reproduce its natural habitat to the maximum and maintain a densely planted aquarium.

Photos of the Harlequin Fish

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