Harlequin Rasbora Care Guide

Written By Lewis German  |  Tropical Fish  |  0 Comments

What are Harlequin Rasbora?

Harlequin Rasbora (Trigonostigma heteromorpha) is possibly the most commonly kept species of rasbora, and for good reason. They are an amazing fish to keep in the home aquarium.

They are part of the family Cyprinidae, which is the largest family of freshwater fishes and includes things like carp, freshwater sharks, barbs and danios.

Harlequin Rasbora are small schooling fish and are highly social, living their lives stuck close together in large groups in and amongst reeds/plant life.

Where are Harlequin Rasboras from?

Harlequin Rasbora are found in a few different places in South East Asia living in forest scrapes, peat bogs and swamps that are saturated with plant life.

They are mostly found in parts of Malaysia, Thailand and Singapore where the waters are tannic and hard to see through.

It is here where they are collected to be shipped off to wholesalers or to be bred and commercially farmed in ponds and totes.

Water parameters for Harlequin Rasboras

Since Harlequin Rasboras come from peat bogs and swamps, there is plenty of decomposing plant material in the water, which dissolves and produces tannins.

Tannic water is brown in colour, almost like tea, and is sometimes called blackwater as it is so hard to see through.

Tannins lower the pH and hardness level of the water and so Harlequins are adapted to survive in acidic soft water.

To keep Harlequin Rasboras healthy and strong, we have found keeping them at these water parameters works best:

pH6.5 – 7.6
HardnessLow to moderate KH and GH
Temperature22C – 26C (71F – 78F)
Ammonia (NH3)0 ppm
Nitrite (NO2)0 ppm
Nitrate (NO3)<5 ppm

Feeding and diet of Harlequin Rasboras

Harlequin Rasboras are a true omnivore and need a range of foods in their diet in order to stay healthy.

They aren’t picky eaters either and often take to dry flakes, pellets and most other prepared and frozen fish foods.

It is good to give them variety, algae wafers for fibre and plant matter, flake food, frozen or live daphnia, brine shrimp and small bloodworms for protein.

Harlequin Rasboras will typically only feed from the top or middle of the tank however, and try not to eat food from the floor if they can avoid it.

Best tank mates for Harlequin Rasboras

Harlequin Rasbora are peaceful in nature. They tend not to fin nip or cause issues for other fish and they are also fast moving and have short fins, which makes them difficult to target for some of the more boisterous or territorial fish.

This means that Harlequin Rasboras can be housed with a wide variety of freshwater fish; however, there are still factors like conflicting water parameters, feeding habits and territory which can make it difficult to house certain fish with Harlequins.

This list contains some of the best tankmates we recommend keeping with Harlequin Rasboras in a community tank:


The staple community fish, Cories make great tank mates for Harlequin Rasboras for many reasons. They like acidic, soft, tannic water, they feed from the bottom of the tank and they are peaceful.

Most Cory Cats pair particularly well with Harlequins as they enjoy the same water parameters and feed along the bottom.

They make great clean up crew, as any food that gets left behind by the Harlequins will be sweeped up by the Cory’s, keeping your tank relatively free from rotting debris.

Black Phantom Tetras

Black phantoms are another small schooling fish that like similar water parameters to Harlequins. They also have very similar diets and eating habits which means they can be kept on the same foods easily.

Kuhli Loach

Catherine Bulinski

Kuhli Loach are another bottom dwelling fish. They are small, brightly coloured snake like creatures that will worm their way around all the cracks and crevices in your tank.

They are great at scavenging bits of food that land between rocks and wood and are constantly active, looking for food.

Khulis also like soft acidic water very much the same as Harlequins; however, they are found in the warmer parts of Indonesia, so they like to be at higher temperatures.

If you keep Harlequins and Kuhli loaches together, keep the temperature at 25C/77F.

Cherry Barb

Cherry Barbs are an excellent starter “nano fish”. They do great in planted aquariums and do very well with Harlequin Rasboras.

They are peaceful, fast enough to not be bullied or eaten and enjoy a similar diet (although may need to be fed smaller food items).

Cherries are also highly adaptable to different water parameters and will do well in normal Harlequin parameters. They will enjoy a heavily planted tank with lots of shade and cover.


Cherry Barbs are an excellent starter “nano fish”. They do great in planted aquariums and do very well with Harlequin Rasboras.

They are peaceful, fast enough to not be bullied or eaten and enjoy a similar diet (although may need to be fed smaller food items).

Cherries are also highly adaptable to different water parameters and will do well in normal Harlequin parameters. They will enjoy a heavily planted tank with lots of shade and cover.

How to breed Harlequin Rasboras

Harlequins these days are bred in commercial amounts in countries like Singapore, where they are housed in large ponds and water vats.

They can also be bred in the home aquarium too if the environment is set up correctly.

Like other members of the Cyprinidae family, Harlequins lay eggs; however, Harlequins will stick their eggs to plant surfaces and leaves to give them the best chances at surviving.

They do not care for their young, and shortly after spawning, they are not strange to consuming their own eggs and young.

To initiate spawning can be difficult, as Harlequins condition up only when kept at very low pH levels, those between 5.0 and 6.5.

Increasing the temperature to around 28C – 82F and feeding much more can also help condition the fish into spawning.

How to sex Harlequin Rasboras

To breed Harlequins, you need both male and female specimens.

To distinguish this can be hard as Harlequin Rasboras are sexually monomorphic, meaning that male and female both look the same to the untrained eye.

There are subtle differences in them however, which are noticeable if you know what to look for:

  • Generally, female harlequins possess broader and more stocky features and have less apparent red colouration.
  • Males are slimmer, smaller and sometimes have more vibrant reds on their dorsal and anal fins.

As it is hard to sex them, your best bet is to acquire a group of around 6 or more harlequins, to ensure that you get a mix of male and female.

Spawning, eggs and fry of Harlequin Rasboras

Once the fish have spawned, they will sometimes eat the eggs and fry, so your best bet to ensure survival is to move the parents or eggs to a different tank.

In this case, it would be better to set up a small breeding tank – specifically to condition the adults to spawn – and then, once they have laid their eggs, move them out.

After around 24 hours, the eggs will hatch into wigglers where they will absorb their yolk for the next 3 days.

After the yolk is absorbed, the wigglers and eventual fry need to be fed on live infusoria and crushed flakes around 2 – 3 times a day until they grow large enough to live with their parents.

Harlequin Rasboras are not one of the easiest fish to breed in an aquarium, but it is certainly doable with perseverance and dedicated care.

What tank size do Harlequin Rasbora need?

Harlequin Rasboras can be housed in something as small as a 10 gallon, although they will never truly thrive as they will need to be kept in smaller numbers and will have their swimming space limited.

For best results, we recommend a 20 gallon tank or larger for Harlequins as it allows you to house higher numbers of them. Around 10 or so and gives them enough space to school properly.

What are the best plants for Harlequin Rasbora?

Java Fern

Harlequin Rasboras do great with just about any aquarium plant as their natural habitat is densely overgrown with bog weeds and semi aquatic plants.

Here are some of our favourite aquarium plants that we recommend keeping with Harlequins:

  • Dwarf Aquarium Lily
  • Anubias
  • Java Fern
  • Bacopa
  • Elodea Densa
  • Brazilian Pennywort
  • Amazon Sword
  • Jungle Val

Tips and tricks for Harlequin Rasbora care

One of our best tips for keeping Harlequin Rasboras successfully is to maintain stability. Harlequins are hardy and highly adaptable to different types of water, but they do not like large rapid fluctuations that change the pH and hardness level dramatically.

Keep your water stable with small frequent water changes and by maintaining a goal pH level of around 6.5.

Harlequins also do very well in heavily planted tanks, with lots of foliage and shade so the more plant life the better.

We could also suggest keeping them in tannic water, although unsightly to some.