Harlequin rasbora care guide

Harlequin rasbora (Trigonostigma heteromorpha) is a small, peaceful and good-looking tank fish. 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

Harlequin rasbora habitats are rivers and streams of the Malay Archipelago, Thailand and Sumatra. The fish was first brought to Europe from Singapore in 1907. 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.

Scientific NameTrigonostigma heteromorpha
Common NamesHarlequin rasbora; harlequin fish; harlequin tetra
Range and habitatMalaysia, Singapore, Sumatra, southern Thailand
Size5 cm (2 in)
Lifespan3 years
Ease of keepingMedium
Minimum tank size40 liters (10 gallons) and more
Temperature75–81 °F (24–27 °C)
Water hardness5-20 dGH


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. The harlequin rasbora body is relatively tall and it is about two inches (5 cm) long with a thin tail-stem. Harlequin rasbora lifespan is about 3 years.

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

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, you can buy 60-80 liters tank (15,85-21,13 gal) for a school of 10-15 fishes.

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.

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

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.

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.

Tank mates

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: cardinal tetra, neon tetra, black neon tetra, dwarf gourami, opaline gourami, betta.

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


Feeding is quite simple since the fish isn’t a demanding one. It can be fed with all types of live and dry feed.

Gender differences: male vs female

You can see difference between harlequin rasbora female and male since the female has more rounded abdomen. The male is more graceful and brighter colored.

As for their black triangular spot on the body sides, the male have more sharpened mark; this mark on the female body is rounded.

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.