Information, Cyprinids, Freshwater fish, Rasboras, Species

Harlequin Rasbora: Complete Species Overview

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by Jason Matthews



The Harlequin Rasbora (Trigonostigma heteromorpha)  is a freshwater fish from Southeast Asia’s tropical regions. It is known for its striking appearance and changing colors. This small fish is easy to care for due to its peaceful and active temperament. It should be kept in groups of eight to ten. Remember that breeding can get hard due to its specific water condition requirements.


Scientific Name: Trigonostigma heteromorpha
Commen Names: harlequin, harlequin fish, and red rasbora
Life Expectancy: 6 – 8 years
Adult Size: 2 inches


Origin Malaysia, Singapore, Thailand, Sumatra
Care LevelEasy-Medium
Breeding LevelDifficult
Temperament Peaceful and friendly
Diet Omnivorous 
Tank LevelMid-dwellers
Water pH73 degrees Fahrenheit to 82 degrees Fahrenheit
Water Temperature5 to 7 
Water Hardness2 to 12 dGH
LightingDim/diffused lighting
Tank MatesNeon Tetras, Cardinal Tetras, Plecos, Platies, etc.

Fun Fact Corner

  • The name ‘Harlequin Rasbora’ was given to the fish due to the presence of the wedge-shaped black pattern. 
  • It was first discovered in 1904 and the first Harlequin Rasbora was sent to Europe in 1906.
  • Harlequin Rasboras love all types of food, but live food is known to make them the happiest!
  • Harlequin Rasboras are one of the very few fish species that swim upside down while mating.
  • Harlequin Rasboras are known as ‘setem’ in the Malay language which means ‘postage stamp’


Harlequin Rasbora
Image: hadot1, Depositphotos

The Harlequin Rasbora is native to Southeast Asian regions such as Singapore, western Malaysia, Sumatra, and southern Thailand. It is a freshwater fish and prefers areas with shallow waters and excessive aquatic plant growth. Therefore, it often inhabits forest streams. 

The Harlequin Rasbora is also found in highly-vegetated swampy areas with low mineral content and high levels of dissolved humic acid. 


Peaceful and easy-going are two words that can clearly define the temperament of Harlequin Rasbora. 

This fish species is one of the easiest to handle as it won’t show any signs of aggression towards other fish species. Some male species might be semi-aggressive, but females are peaceful. 

Harlequin Rasbora is a shoaling fish, so it prefers to stay in a school (or group) of eight to ten. In groups, it is extremely active. On the other hand, a single Harlequin Rasbora might become shy and aloof.

This fish can also easily become the target of large, predatory fish due to its attractive and shiny coloration. 


Harlequin Rasbora requires specific tank conditions to thrive and live healthily. Typically, the lifespan of a Harlequin Rasbora is between five and eight years.

Most Harlequin Rasbora live up to six years if proper tank conditions are provided. 

Size and Appearance

Harlequin Rasbora, rasbora heteromorpha, Aquarium Fishes

Harlequin Rasboras classify as small fish as they can only grow up to two inches in an aquarium setting. However, usually, most of them only grow up to 1.75 inches due to several factors. 

Body Shape 

Harlequin Rasbora’s striking appearance makes it distinguishable amongst other fish species. It consists of a diamond-shaped body with more weight in the center. Towards the head and the back, the fish’s body tapers into a thin shape.

Body Color

Harlequin Rasboras are known to have beautiful body hues such as orange, light pink, red, and golden

Many aquarists have often noticed that their Harlequin Rasbora changes color throughout the day. According to them, this fish has a paler body color in the morning that becomes more vibrant as the day forwards. The colors can change as per the varying water conditions too.

The entire body of Harlequin Rasbora is covered with fascinating colors. The head and the tail region have more defined colors whereas the belly area tends to be white. 

When it comes to the fins, this fish has colored dorsal and caudal fins whereas the pelvic and anal fins have a more transparent appeal.  

Depending on the body hues, you will find many varieties of Harlequin Rasbora. 

  • Golden Harlequin Rasbora
  • Black Harlequin Rasbora 
  • Blue Harlequin Rasbora 

Unique Feature – Dorsal Patch 

The one standout feature of Harlequin Rasbora is the dorsal patch. It is a type of wedge-shaped patch of black color that starts at the dorsal fin and tapers at a point at the base of the caudal fin. 

The male Harlequin Rasbora has a slightly rounded dorsal patch whereas the female has a straight dorsal patch. 

Differences Between Male and Female Harlequin Rasbora

The table below indicates the differences between male and female Harlequin Rasbora. 

CategoryMale Harlequin RasboraFemale Harlequin Rasbora
SizeSlightly smaller than femalesTypically larger and girthier
Body ColoringIncreased red coloring in the dorsal, caudal fin, and caudal peduncle regionFemales have a more golden tint 
Shape of Dorsal PatchThe dorsal patch is more rounded towards the bottom with an elongated tipThe dorsal patch is straight rather than rounded


Although Harlequin Rasbora is easy to take care of, you cannot say the same for their extremely tricky breeding process. 

You need to provide specific tank conditions to initiate any sort of breeding. Below is a quick list of everything you need to do to breed Harlequin Rasboras.

  • Start with having a separate breeding tank. 
  • Add peat moss to mimic the humic acid in the fish’s natural environment. 
  • Increase the temperature to 80 degrees Fahrenheit. Also, add some broad-leaf plants for the female to lay eggs. 
  • Add a group of Harlequin Rasbora to the tank. Ensure that there are two females for every male.
  • Wait for the female to find a leaf and call the male to fertilize the eggs. 
  • The fertilized eggs are typically in a group of twelve and will then attach themselves to the underside of the leaf.
  • Remove the adult fish and transfer them back to the original tank.
  • The fertilized eggs will grow into baby fish in 24 hours.

Nutrition and Diet 

Harlequin Rasbora is omnivorous in nature. Most of them aren’t picky eaters, and will gladly eat frozen, granules, good-quality flakes, pellets, and live food. Occasionally, they enjoy vegetables too. 

The table below shows some of the best food options for Harlequin Rasbora. 

Type of FoodBest Options
Frozen/LiveBrine shrimp, blood worms, black worms, daphnia, fruit flies, grindal worms
Live FoodBrine Shrimp, daphnia, mosquito larvae
VegetablesAlgae wafers, zucchini, cucumber pieces

You should provide your fish with food at least five to six times a week. 

Note that Harlequin Rasbora has a small mouth. So, try to crush the food to ensure that the food pieces are tiny and easily consumable by your fish. 

Tank Requirements

Harlequin Rasboras are mostly mid-dwellers and don’t like to venture to the top or bottom of the tank. Therefore, it is best to get a tank with a wide bottom. This way, they can swim freely. 

Tank Size

The tank size depends on the number of Harlequin Rasbora fish that you’re planning to get. However, you require a minimum 10-gallon tank to house Harlequin Rasbora. 

As this fish is better in groups of six to eight and loves to swim around, it is best to opt for a 20-gallon or a 30-gallon tank

Water Requirements

Water requirements of Harlequin Rasbora are quite strict. This fish requires specific water conditions to live a healthy and long life. 

  • Water Temperature – As Harlequin Rasboras are native to tropical Southeast Asia, they require warm water temperatures. It shouldn’t be too high or too low. The ideal temperature for this fish ranges between 73 degrees Fahrenheit to 82 degrees Fahrenheit.
  • Water pH – Harlequin Rasboras are extremely vulnerable to changing pH levels. So, ensure that they remain constant. This fish requires a slightly acidic to neutral pH ranging between 5 and 7. 
  • Water Hardness – Harlequin Rasboras require soft water as their natural habitat consists of that. The water’s hardness should be kept between 2 and 12 dGH.
  • Water Flow – The water flow should be slow for this fish species. Too high water flow or dirty water can deteriorate your Rasbora’s health.

Decoration and Plants

Harlequin Rasbora is a social fish. Yet, when it feels anxious or stressed, it prefers to hide behind the plants. 

So, try to keep items like driftwood, rocks, and plants in the tank to simulate the fish’s natural habitat. Some of the best plant options include Vesicularia, Weeping Moss, Rotala, and Ludwigia.


Harlequin Rasbora doesn’t require any additional lighting. It is best to provide them with diffused light or dim light in the aquarium to mimic their natural conditions. 

Best Tank Mates

Harlequin Rasbora blends well with its own species. However, it can also get along with various other fish species. These include: 

Common Diseases In Harlequin Rasbora 

Harlequin Rasboras are susceptible to diseases if they’re kept in improper pH conditions, extreme temperatures, or dirty water. Some of the common diseases in this fish species are given below. 

  • Columnaris
  • Fin and Tail Rot
  • Viral Haemorrhagic Septicaemia
  • Dropsy
  • Pop-eye Disease
  • Mouth Fungus
  • Fish Fungus / Fungal Infections
  • Ich
  • Anchor Worms 


Jason Matthews

My name is Jason Matthews, and welcome to my website. When other kids were bragging about how their dog could sit and roll over, I was bragging about my latest Betta Fish and the cool sea castle I just added to his aquarium. 

Jason aquariume

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