Harlequin Rasbora: Species Profile

Category: Barb

Common Names: Harlequin Rasbora, Red Rasbora, Harlequin Fish

Scientific Name: Trigonostigma heteromorpha

Family: Cyprinidae

Minimum Tank Size: 20 Gallons

Care Level: Easy, but sensitive to water parameters

Temperament: Peaceful

Max Size: 2 inches

Temperature: 72-80 F

pH: 5.5-7.0

Tank Level: Middle to Top

Colors: Orange, Silver, Black Red

Diet: Omnivore

Breeding: Moderate

By Stefan Maurer [CC BY-SA 2.0]

By Stefan Maurer [CC BY-SA 2.0]

Species Overview

Named for their unique pattern on the lower half of their body, Harlequin Rasboras have been a popular aquarium fish since their introduction in the 1900s. Their reddish coloration has made them a favorite for freshwater tanks. The shoaling fish are most comfortable in groups, creating a striking visual as they explore their habitat.

Known for their non-aggressive nature, Harlequin Rasboras live peacefully with many other species. Tank set-up for the fish is relatively easy, making it an excellent fish for beginners. If you’ve already got a setup, a group of Harlequins can also be a great addition to already established tanks.

Distinguishing Features

Harlequin Rasboras have several distinguishing features that set it apart. Features you can look for include the animal’s distinct pattern and coloration but also its shape.

Rasboras are part of the Cyprinidae family, consisting of carp and minnows that live in freshwater. Their scientific name is Trigonostigma heteromorpha. Heteromorpha in Greek means “differently shaped”. This name refers to its difference in shape compared to other species of the same family.

While the fish’s bodies are silver, the fins are red. The reddish color varies from peachy pink to golden orange to darker red hues. Harlequin Rasboras’ caudal fins are forked.

A black triangle starts at the dorsal fin and tapers down toward the tail. Many people refer to this black area as the black wedge. The black wedge’s shape can vary between genders.

Black Harlequin Rasboras are a popular color morph of the species. Other morphs include purple, blue, and gold. These color variations don’t influence behavior but might make them more sensitive to tank parameters.


Harlequin Rasboras were first imported from Asia in the 1900s. The fish originates from the area surrounding the Malay Peninsula. On the Indonesian island of Sumatra, Harlequins are called “the triangle fish.”

Harlequins live inland in freshwater wetlands. Often found in streams and other waterways that flow through peat swamp forests, Harlequins thrive in areas with lots of plant matter. These waterways have relatively moderate or low currents.

The water in these areas is clear, not cloudy. Leaves and other organic matter stain water, making rivers brown or yellow in appearance. Streams Harlequin inhabit have soft water and may be slightly acidic due to the peat.

Waterlogged soil in these areas stops decomposition resulting in a layer of peat. Very few peat swamp forests exist in the world. The unique habitat is also home to endangered species like tigers, elephants, and clouded leopards.

Human development threatens the peat swamp forests where Harlequins live. Pollution from deforestation and agriculture are two of the main concerns. Habitat loss also threatens the fish.

Overfishing for the pet trade is also a concern. However, the fish are usually caught by hand during periods of low water, overfishing is not considered a threat. Since Harlequin populations collapse during these low water periods, collecting them this way isn’t particularly harmful.


Harlequin Rasboras typically range in length from around one to one and a half inches. They can sometimes grow larger but are never more than two inches long.

Female Harlequins are usually a bit bigger than males. They are often more round as well. The roundness will be especially apparent when females are carrying roe.


Harlequins usually live to be around five to eight years old. Some care guides state that these fish will only live to around three or four years old. Proper care of Harlequins will help to ensure their longevity.


Identifying differences between male and female Harlequins includes body shape and patterns. Telling the two apart can be difficult at times, but if you look for these traits it will be easier.

Males, as mentioned previously, are usually slightly smaller than females. The black wedge pattern is more defined, and the fins are more brightly colored.

Females are larger, and their body shape is more rounded. The black wedge pattern tends to be more rounded in appearance on females.


Harlequin Rasboras are well-known for their peaceful nature. Their temperament makes them ideal members of community tanks. If you’re considering adding Harlequins to your existing tank, remember to evaluate the temperament of your other fish.

You should keep Harlequins in groups of at least six, although eight is another standard recommendation.Shoaling together helps Harlequins feel calmer in their environment. Fish shoal together not only for safety but also to locate food more quickly.

Keeping Harlequins together helps them display their best colors as well. Males display more color when competing for females. There should be two females for every male in the tank for the best results.

I, Lerdsuwa [CC-BY-SA-3.0]
I, Lerdsuwa [CC-BY-SA-3.0]

Tank Parameters

An established planted tank is the best way to start with Harlequin Rasboras. Before setting up a Harlequin-friendly aquarium, check your tank parameters to make sure the it meets the needs of your Harlequin

To help your Harlequins thrive, ensure the tank mimics the tropical forests where they live. Temperature, pH, and water hardness are essential factors in your tank.

Minimum Tank Size

The minimum recommended tank is ten gallons. A tank this size would be appropriate for groups of Harlequins of around six to eight. Make sure your tank is at least ten gallons before adding your new fish.

A more extensive shoal requires more room. Tank setups for multiple species should also use larger tanks.

A general rule of thumb most aquarium keepers follow is for every inch of fish, there should be a gallon of water. This simple equation isn’t all-encompassing as many other factors affect how many fish can live in a tank. Other things to consider are the types of fish and the power of the filter used in the tank.

Water Parameters

Your tank for Harlequins will need to be heated. Maintaining a neutral pH is also essential.


Set water temperature within 72° to 81° for proper heating. Though Harlequins can tolerate a range of temperatures, keep the temperature consistent. Use a tank thermometer to monitor this parameter.

Keeping Harlequins with other tank mates means considering their needs as well. Remember this when establishing the temperature for a tank.


The ideal pH range for Harlequins is 5.0 – 7.0. They will tolerate slight variations, but it’s best to keep their tank within these levels. Monitor pH frequently to ensure the health of the aquarium.


Harlequin Rasboras are freshwater fish and do not require salinity in their tanks.

Water Hardness

The hardness of water fluctuates depending on its mineral composition. Water passing through rocks picks up some of those minerals. Waterways making contact with limestone and dolomite are more likely to be hard.

In the wild, Harlequins live in soft water. Monitor fish tanks with Harlequins to ensure they stay within the 2-15 dGH. dGH refers to degrees of general hardness. Measure the hardness of a tank with a water hardness testing kit at least once a month.

Tank Setup

Your Harlequin Rasboras require tank setups with live plants where they can hide. These fish tend to stick to the middle and top of the tank and also need some open space for swimming. Here are all the things you’ll need to know about prepping your tank for Harlequin Rasboras.


The substrate for a Harlequin tank should benefit the plants in your tank. Harlequins don’t spend a lot of time on the bottom of the tank and don’t have specific requirements for the substrate.

Many aquarium keepers opt for a dark substrate because of the color contrast, but a sandy substrate is also suitable. Gravel will also work also long as plants can take root.

In addition to substrate, adding leaf litter to the tank’s floor helps recreate your Harlequin’s natural environment. As the leaves decompose they will release additional nutrients into the tank.


The decorations in a Harlequin tank should be close to what they would experience in their natural habitat. You should create places for them to hide out with rocks and driftwood in addition to plants.

Always wash any decorations before placing them in an aquarium. Never bring decorations in from outside to use in your tank. Even washing natural wood won’t remove all of the parasites that could be residing there.


The best plants for Harlequin Rasboras are from regions they inhabit in nature. Members of the genus Cryptocoryne are great additions to the tank. There are many species of Crytocorynes to choose from, and they are very versatile.

When opting for Cryptocoryne bear in mind that most plants will go through a process called Cryptocoryne Melt. Melt occurs when the first leaves on the plant seemingly melt away.

The process of Cryptocoryne Melt happens because many aquatic plants are farmed above water for the ease of the growers. These submerged leaves will often become translucent and fall away. As long as the plants are growing within normal parameters new leaves will form.

Don’t move plants affected by this melt. Doing so could cause the roots more stress, which won’t benefit the plants. The problem usually fixes itself but trimming damaged leaves can help encourage healthy growth.

Look for other plants to add to your tank that grow well in dim lighting. Other plant families to look at include Microsorum, Taxiphyllum, and Anubias. Consider floating plants for the top of the tanks to help filter light.


Harlequin Rasboras don’t require a lot of lighting. They are obscured from the sun by vegetation in their habitat in nature.

Harlequin tanks still require lighting for the plants to survive. Using a low output light will probably be best for the plants and fish living in the aquarium.


A filter is necessary for tanks to ensure a clean environment for the creatures living inside. It’s recommended that you use a filter that handles five to seven times more than the water volume.

The purpose of a filtration system is three-fold. Filters trap debris and remove debris from tanks. Activated charcoal in filters removes substances already dissolved in the water. Finally, filters convert ammonia to nitrites.

When considering adding Harlequins to an existing tank, try using this tool provided by AqAdvisor. This aquarium calculator determines the filtration capacity of the filter compared to the size of the tank and its inhabitants. They also provide information on recommended water change schedules and aquarium stocking levels.

Filtration and tank size will determine if adding Harlequin Rasboras to a current tank setup is recommended.


A heater will be required to get the desired temperature for Harlequins to live. There are several types of water heaters available so choose one that is the most suitable for your setup.


Harlequins feed on worms, insects, and other small invertebrates. They are considered micro predators and can be very excitable when eating. These fish are also prone to jumping, so be especially careful during feeding time. If you choose Harlequins for your tank make sure you have a lid to keep them safe.

In captivity, Harlequins should be fed a variety of food items. A mix of flakes or pellets and live or frozen items like bloodworms or Daphnia (sand fleas) provides differing protein sources. A mixed diet will help Harlequins display their best colors.

Like other aquarium fish, Harlequins should be fed once or twice a day. The amount of food given will vary on the number of fish in the tank. If the food provided isn’t gone within a few minutes, the amount provided may be too much.

Overfeeding our pets may feel like a reward for them, but often it does more harm than good. Overfeeding in fish causes stress to the body and their environment. Leaving extra food in the tank can create excess ammonia and not all filters are built to handle this build-up.


Occasionally Harlequins will breed spontaneously, especially if the fish are healthy and there are plenty of broadleaf plants where females can lay their eggs. Despite this, getting your Harlequins to reproduce can be difficult, but breeding fish at least one-year-old yields better results.

If you want to attempt breeding your fish, get Harlequin Rasboras ready to spawn by feeding them a protein-rich diet. You should provide live or frozen prey two to three times a day during the weeks leading up to breeding.

For best success when breeding, keep your Harlequins in a separate tank from where they usually are held for best success. This tank should be kept dim and have plenty of plant cover. The tank will require a heater, but a filter will not be necessary until the eggs have hatched.

Place your male and female in the tank overnight, and they will usually start courtship behavior in the morning. Expect the female to lay eggs shortly after this behavior. Female harlequins float upside down next to broadleaf plants to lay their eggs; the male fertilizes them.

If your fish don’t spawn right away, try leaving them in for a few additional days but no more than four days. Once the fish have spawned, remove them from the tank as they will eat eggs.

Harlequin Rasbora eggs hatch as fry in as little as 24 to 48 hours. Initially, the fry should be fed infusoria, or tiny microorganisms, until they are big enough to consume other fry food like microworms.

Common Diseases

Harlequin Rasboras are not particularly susceptible to disease but still can become sick if not cared for properly. These are some of the most common afflictions in freshwater aquarium fish that could affect Harlequins.

  • Fin Rot causes include bacterial or fungal infections and even aggressive tank mates. The fins of the fish with begin to disintegrate. Rot can affect the tail as well.
  • Ich, short for Ichthyophthirius multifiliis, is common worldwide. Ich refers to the white spots that look like grains of salt covering fish infected with this parasite.
  • Dropsy is a bacterial infection that attacks the kidneys causing fish to bloat. Dropsy is rarer than the other two conditions.

Keeping aquariums clean and closely monitoring your fish for changes is vital to preventing most diseases. Treatments are available for some common diseases among Harlequin Rasboras. Consult with a veterinarian before beginning any treatments.

In very rare cases, fish can spread disease to humans. These are primarily bacterial infections. Take care when cleaning tanks with potentially sick fish.

Potential Tank Mates

Harlequin Rasboras coexist with many different species including other rasbora species like scissortails.. Their temperament is peaceful, and their keeping needs are compatible with many other types of fish.

The most important thing for you to consider, outside of tank parameters, is the size of the fish kept together. Even species living together might make a Harlequin Rasbora a snack if it outgrows it. Choose fish that are less than double the size of Harlequins.

Temperament is another factor to evaluate before making other species into Harlequin tank mates. They don’t care for overly energetic or aggressive behavior.

More populous shoals of fish provide a sense of security. In these larger groups, they are less likely to feel bothered by the behavior of other fish.

Species to native to the same region are good tank mates to consider. These include the Lined Barb, Five Banded Barb, Redstripe Rasbora, and Eyespot Rasbora. Other species sharing similar habitats are also a good fit.