The Harlequin Rasbora, scientifically referred to as Trigonostigma heteromorpha, is a freshwater water tropical fish that originates from Southeast Asia. They were found in Thailand, Singapore, Malaysia, and Sumatra. They are among the most popular schooling fish across the globe. This fish has a shimmering coloration which is accompanied by their iconic marking hence producing the beautiful show in the aquarium.

This fish also prefers living in fresh blackwater streams and pods. This species is loved by most aquarists due to the ease of taking care and their stunning looks. Watching them in a school as they swim can be a memorable moment.

Roel Balingit, CC BY-SA 4.0, via Wikimedia Commons

This fish species was first discovered over 100 years ago, and since then, they have been a great commodity in the aquarium market. This has mainly been as a result of their appearance and their hardiness, and this makes them the ideal choice for beginners since any level of skills is okay.

This species is also very peaceful and will easily get along with other peaceful species. They can, however, be eaten by large species, and that is why their tank mates should be of the same size. They are usually readily available and usually high on demand, and therefore most stores will sock them.

As an aquarist, you need to have information about all you need to know and do in the process of keeping this species, and that is what the guide seeks to do.


Scientific NameTrigonostigma heteromorpha 
Care LevelEasy
ColorSilver body with a black patch and orange fins
SizeUp to 2 inches
Lifespan5 to 8 years
Tank size10 gallons
Freshwater or SaltwaterFreshwater
Temperatures71 to 80 degrees F
CompatibilityPeaceful community aquarium


The Harlequin Rasbora is a peaceful fish that does not bother other fish species. You can choose a small fish as the tank mate. You should be very careful not to choose large species that are bigger than the size of this species since they will eat them. Cichlids are examples of those species that are likely to do that.

Boisterous fish can also be a problem to this fish. This is because the fish are timid, and they are likely to get stressed if they are chased by over-reactive fish. You should avoid some species such as bettas and clown loaches since they may harass others.

The ideal tank mates for this fish species are the small peaceful fish such as;

  • Guppies
  • Cherry barbs
  • Mollies
  • Dwarf Gourami
  • Corydoras catfish
  • Hatcher fish
  • Danios
  • Tetras
  • Platies
  • Zebra loaches

The aquarium invertebrates are also gaining some popularity over time, and you can keep snails and shrimps with this species. They are unlikely to notice each other. You can take a try on some mystery snails, Amano shrimps, or cherry shrimps. The adding of the invertebrates inside these tanks helps to diversify the tank. They are also very good at cleaning up after your fish too.


Being peaceful species, these fish can be kept together. In fact, you should keep them in groups of 4 and above. They are shoaling fish that do well in a group of their own kind. The bigger the number, the better the performance of the display will be. You should, however, be careful about the number you are keeping against the size of the tank available to avoid overcrowding.


Mariusz Dabrowski, CC BY-SA 3.0, via Wikimedia Commons

This species rarely grow beyond 2 inches in length. They are generally small in size, and they can fit a few of them in one tank. Their body has a tall mid-section but narrows towards the mouth and a forked caudal fin. The rear half part of the body contains a patch that is black in color and narrows with the body as it stops at the caudal fins.

It is out of this black patch that this fish got its name is similar to the patterns that are found on a classic harlequin outfit. The rest of the body is silver with a tinge of orange. The fin has a darker orange, but it can depend on many factors such as the tank conditions, the levels of stress of the fish, and the population they were bred from. These patterns are not striking on their own, and therefore a group can create quite a spectacle.

As a result of selective breeding, this fish can also be found in a few other designs. The black Harlequin Rasbora has a large black patch on their body that reaches up to behind the head. This species has a less shouting black patch, and their scales are tinted violet. There are other colors that have been bred too. The other designs are less popular, and this means that you may have to look around for a while if you are interested in them.

It can be a bit challenging to distinguish between a male and a female. However, the males tend to have a slightly larger patch and a more rounded area where the anal fin attaches. You can easily mistake this species for the Lambchop Rasbora since they resemble each other. Both of them have a unique black marking, but the Lambchop Rasbora has a brighter orange around the body edges.


This species grows to a maximum size of 2 inches in length in captivity. When fully grown, they do not get very large beyond that. This size is the higher end of the spectrum, but most of them will grow to an average of 1.75 inches. This makes it the ideal choice for a small tank.


This fish species originates from South Asia across the Malay Peninsula region in Singapore, Thailand, Malaysia, and Sumatra. They prefer living in streams, rivers, and swamp forests. The water in the natural habitat contains high levels of dissolved acid and low mineral levels. This is the kind of water that you will mainly find flowing in the peat swamp forest. This generally indicates that the water resembles the black water habitat of South America.

The water has a yellowish appearance that is a result of tannins. This fish loves the presence of high vegetation in the habitat, and this provides the much-needed shade that they need. In the wild, this species lives in soft, slow-moving, calm,  low acid, and neutral water; most of the Harlequins that are resold in the market can adapt to a wide range of water since they are raised in a fish farm in general.


On average, the lifespan of this species ranges from 5 to 8 years under optimum care. The length of their life is determined by various factors such as the water conditions and the diet they are fed from a young age. If you want your pet to attain this age, you need to provide a clean and healthy environment to live in, and the diet you are feeding them should be nutritious and enough.


This species is a middle section dweller. They love spending most of their time in this section. They are rarely found swimming near the bottom section near the substrate, and therefore as you look for tank mates, you need to consider those that love residing in that section. They’re peaceful fish that will not bother the other fish in the tank. This fish has no history of fin napping, and this makes them a great choice for those who want to introduce more color into the tank without having any effect on the overall community.

This species is commonly known to be kept in a species-only tank, but it is not a requirement. With their peaceful nature with no aggression, they can perfectly live in a community tank. The main reason why most aquarists do not put them in the community tank is due to the fact that they cannot defend themselves from harassment. They are easily a target of the bigger and more aggressive fish, and this leaves them vulnerable, and they can easily pick injuries that can be fatal.

This species loves living in big groups, and it is very crucial that they are within a big group as they would otherwise not live happily. They mainly do not live alone, and they mainly depend on the company of others to avoid getting stressed and anxious. This species should be kept in a group of 8 to 10, and they will perfectly fit in a ten gallons tank due to their small size.

When you keep this species in a school, they are more likely to show vibrant color and personable behaviors in large groups. Basically, these fish need to be kept in large numbers so as to thrive best.


The process of taking care of this species is quite easy despite the experience you have, and this is great news to every potential owner. This fish is a bit hardy can be able to tolerate slight fluctuation in temperatures. This does not mean that you can just add them into any tank. Just like any other species, they have some conditions that you should provide in your tank for them to thrive. To enable your Harlequin Rasbora to thrive to its full potential, below are some of the conditions you should meet;


The minimum size of the tank for this species should be 10 gallons. This is enough to house a number of them. This size is enough assuming you are keeping a small group; do not overcrowd. As a rule, it is advisable that you keep a pair of this species for every gallon your tank holds. If you have a bigger space, it is recommended that you go for a bigger tank since they are easy to maintain. A tank that is 20 to 30 gallons will give our fish enough space to swim and relax.


This species does not need a strong filtration system. All you need is a common air-powered sponge filter or a peat filter, and they will effectively keep the water clean and aerated. This fish does not need a strong current but rather low currents, and that means that you do not need to look for some high-end filter for them.


This fish species is not too picky about the type of substrate that you will pick. However, the main determinant of the substrate that you will choose is the type of plants that you will put in the tank. If you are interested in introducing live plants in your aquarium, the ideal substrate should be gravel-based since this will help the plants to root easily.

You can also go for sandy substrate, but the recommended type is the play sand instead of the construction one. Whatever types of substrate you will settle on, ensure that it is dark in color s as to blend with the dim ambiance.


The recommended amount of lighting for the aquarium should be moderate to subdued because providing them with bright light would force them to retreat to the hiding spots. Also, the dim environment helps bring out the bright coloration of this species. Remember that in their natural habitat, they love a heavily planted environment so that the shades can provide hiding spots and dim some light through shadows.


This species loves hiding sometimes, and therefore it is crucial to provide the hiding spots. You can provide aquatic caves and rocks, and they will perfectly serve as hiding spots. These decorations should not have sharp edges and rough surfaces that might injure the fish as they swim or hide.


The presence of heavy vegetation in the tank is very crucial since it allows the vibrant colors of the fish to shine. However, you should be careful not to overplant the aquarium and deny the fish enough swimming space. It is advisable to have the broad-leaved plants so that they can act as shelter and hiding places and provide shade in the aquarium.


It is advisable to clean the tank on a monthly basis so as to ensure the living environment of your fish I clean and healthy. Providing a clean tank to this species will consequently add more life to them. As you clean the tank, you should ensure that you do not use any soap-based or chemical products since they are very harmful and toxic. You should instead use Luke warm water and a piece of cloth. To clean the substrate, you can use a gravel vacuum.


As said earlier, this fish is generally hardy and can handle changes in water conditions. This does not, however, mean that you should keep changing since this will stress the fish and even affect their health. You should try as much as possible to keep the parameters within the recommended levels throughout. Some of the requirements include;


Harlequin prefers living in warm water, which mimics their natural habitat. The average temperature should range between 72 to 81 degrees F. To ensure that the water remains warm throughout, you can install a heater in the tank and a thermometer to keep monitoring.


The PH level should range between 5.5 to 7.0 for your fish to thrive. This species opts for acidic water over alkaline water, and therefore you need to use peat moss in the filter in order to maintain acidity and increase the carbon by adding more plants.


The water hardness should be minimized as much as you can, and it should be kept between 1 to 10 dGH. You can add some chemicals to the tap water in order to minimize hardness and remove calcium and magnesium. Driftwood is mainly used in removing water hardness.


Water should be changed on a regular basis so as to ensure that the tank is clean and healthy. You can choose different intervals as an aquarist and the amount of water to replace depends on the interval you choose. You should never change the water in its entirety since this will kill the beneficial bacteria. The other reason why the water should be replaced is to remove all the uneaten food from the tank. This will be important in reducing nitrates in the tank and preventing the growth of algae.

There are three intervals which you can choose from. The first is the weekly interval, and in this, you should replace 10 percent of water in the tank. The second interval Is the fortnight interval, and if you settle for this, you should replace about 20 percent of water. The last is the month when you need to replace 40 percent of water in the tank.

You also need a water testing kit in your tank so as to keep monitoring the concentration of ammonia, phosphates, and nitrates in water. This exercise should be carried out every 15 days. After finding the high concentration of these components, you should always take a step in trying to reduce them.

You should also not add the fish in the tank before you have performed the nitrogen cycle for between 2 to three weeks. The uneaten food should be removed immediately so as to avoid excessive nitrate and ammonia, which affect the cycle.

The new batch of water that you are adding to the tank should maintain the water parameters as they should be. The temperatures, PH, and hardness.


These fish are Omnivores in nature, and they feed on a wide range of food, both plant-based and fresh-based. You should, however, be careful about the size of the food that you feed them once they are small in size. They have a small mouth, and this means that the food you are providing should be in small particles. In the wild, the Harlequin Rasbora is restricted to foods like plant detritus, small insects larvae, and eggs. They are scavengers and will feed on anything that they are provided with.

The best means of feeding this species is by feeding them on store-bought foods. During the feeding times, they become active and will take the pellets or flakes happily from the surface. These foods are the best since they are designed to provide a wide range of nutrients. It is always important to provide a varying diet not necessarily for the sake of nutrients but also to keep it interesting by breaking the monotony.

This can be achieved by feeding them live or frozen food. Some of these examples are bloodworms, daphnia, and insect larvae. The live foods make this whole process of feeding interesting and fun by introducing movements. This species will sometimes feed on the plants in the aquarium, but it is something you should not be worried about. This implies that you can sometimes feed them on vegetables or use the vegetables to make your own homemade food.

This species should be fed twice a day, and you should give them a small amount of food that they can be able to finish in minutes. Remember overfeeding can be dangerous to their general health. Feeding them in small amounts also gives their digestive system an easier time. After feeding, they ensure that you remove all the uneaten food from the aquarium to prevent decay.


The Harlequin Rasbora is not so easy to breed. They are egg layers, and it is the female that deposits the eggs on the underside of broad leaves, and the male then proceeds to fertilize them. In order to encourage breeding among this species, it is advisable to separate the breeding pairs in a breeding tank with a ratio of two females to one male. You can also start feeding them with live food.

You should have the broadleaf plants in your tank since the leaves are useful in providing the place for laying eggs. The breeding happens during the early morning hours. To encourage spawning, you can raise the water temperature slightly for some days. The water hardness should not go above 4 dGH, and the PH levels should be around 6.4.

 After the eggs have hatched, you should remove the adults from the breeding tank so as to give the newly hatched fries a survival chance. They will stay in a separate tank until they are at the age of 6 to 9 months. Thereafter they can join the community tank. This ensures they are big enough and they cannot be eaten by the big species, and during this time, the tank cleanliness should be observed.


The Harlequin Rasbora is hardy fish species that originally lives in streams and swamps, and therefore they do not get sick easily. However, just like any other fish, they are susceptible to common fish diseases. Some of the diseases include;


This is the most common disease among this species.   You should treat it as soon as you notice it since it destroys vital organs such as the liver, lungs, and brain if left untreated. Some of the notable signs of this disease include; patches on the fins and skin, a slimy layer on the surface of their skin, and the skin turning brown.

To treat this disease, you can isolate the affected fish from the tank mates to avoid spreading the disease. Then add the aquarium saltwater, get rid of chlorine water from the tank, and increase the temperature to about 77 degrees F. You can also try over-the-counter medication like the API Fungus Cure.

  • ICH

This is another common disease among most fish species. It is also known as white spot disease. This is where the external parasites cling to the body of the fish and end up causing white spots on the skins and gills. It is caused by a parasite in contact with other infected fish. Also, sudden changes in water temperature in the tank and the decorative plants that may have parasitic cysts in them.

Some of the common signs include crystal formation on the body, rubbing against the surfaces, difficulty in breathing, skin becoming slimy, and hiding for days. You can treat this disease by raising the water temperatures slightly, and you can also use the medication if the problem persists.


Basically, this is kidney failure. Water and other fluids accumulate in the body and cause swelling in the abdomen. This disease is mainly caused by water with excess ammonia and nitrates, and some bacteria also. Some of the signs include; gills losing coloration, irregular swimming, bloating belly, scales standing out, and stringy feces.


This is another common disease among this species, and it occurs by the tissues of the tail start to rot outwards. The rotting might start from the wound that your fish has already, and it may also be as a result of dirty water. This is mainly caused by overfeeding, high-stress levels, dirty water, and low nutrition foods.

Some of the signs include inflammation of fins, and then they start losing color, and they finally start becoming frail. You can clean this disease by providing clean water, optimizing the temperature and PH of the water.


This is a highly contagious disease that only affects freshwater fish. It is caused by low oxygen levels in the aquarium and the temperatures in the tank. Some of the notable signs include; mucus on the gills and dorsal fin, ulceration on the skin, loss of appetite.

To treat this disease, you need to stabilize the water parameters such as temperatures. You can also consult a vet if the problem persists.


Is the Harlequin Rasbora Freshwater fish?

Yes, this is a freshwater fish that originates from Southeast Asia. They are mainly found living in rivers, streams, and swamp forests.

Does the Harlequin Rasbora need a heater?

Yes, it is important to provide a heater in the tank so as to warm the water whenever you notice a drop in the water temperatures. Ensure you fix it in the tank and a thermometer to monitor.

Can Harlequin Rasbora Live alone?

Yes, this species can live alone in the tank, but they will thrive best in a group. They are shoaling fish, and they are active and happy in a group. It is advisable that you keep them in groups of 8 to 10.


Generally, the Harlequin Rasbora is hardy fish that needs little attention to thrive. This makes it the ideal choice for beginners. This, however, does not mean that you place it in the tank and do not bother taking care of it. No. It is important to follow all the conditions discussed in the guide, and your fish will thrive and live a long, happy life.

It is our hope that this guide will be resourceful to you and has given you detailed information about the care of this fish. You should consider having this species in your tank, and the experience will be priceless.

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