Skip to Content

Harlequin Rasbora (Trigonostigma Heteromorpha) Care Guide

Harlequin Rasbora (Trigonostigma Heteromorpha) Care Guide

Originating from Southeast Asia, specifically in the clear, slow-moving waters of Malaysia, Thailand, and Sumatra, the Harlequin Rasbora is an undoubtedly captivating freshwater fish. Its distinctive appearance and lively demeanor make it a standout addition to any aquarium.

This article will provide you with valuable information on creating the ideal environment, maintaining water quality, and fostering the health and happiness of these stunning fish in your home aquarium

Key Takeaways

  • They need at least a 10-gallon tank with lots of plants, and they prefer groups of at least six.
  • Feed them live or frozen foods like brine shrimp and bloodworms along with flake food for a balanced diet.
  • These fish are peaceful so choose calm tank mates; avoid large, aggressive fish that might harm them.
  • Regularly check the water quality and watch for signs of sickness to keep Harlequin Rasboras healthy.

Harlequin Rasbora Overview & Natural Habitat

  • Common name: Harlequin Rasbora
  • Scientific name: Trigonostigma heteromorpha
  • Care level: Easy
  • Size: Up to 2 inches
  • Lifespan: 5 – 8 years
  • Temperament: Peaceful
  • Diet: Omnivore
  • Group size: Groups of 6 to 8
  • Minimum tank size: 10 gallons
  • Water temperature: Between 76°F and 80°F (24°C – 26°C)
  • Water pH levels: Between 6.0 and 7.8
  • Water hardness: 2-15 dGH 

The Harlequin Rasbora, scientifically known as the Trigonostigma heteromorpha, is a charming little dweller of Southeast Asia‘s tropical waters. This diminutive and iridescent fish calls the forest-shaded streams and rivers of Malaysia, Singapore, Sumatra, and southern Thailand its home.

In their native habitat, these agile inhabitants naturally gravitate towards waters imbued with humic acids, the lifeblood of blackwater ecosystems. These acids seep steadily from decaying plant matter, crafting an acidic haven that these Rasboras relish.

Familiar with the tea-colored currents of peat bogs and densely vegetated wetlands– where daylight gently filters through layers of treetops their presence brings an effortless tranquility to any aquarium space.

Appearance & Size

The Harlequin Rasbora boasts a striking appearance with a streamlined body adorned in vibrant colors. 

Its sleek physique, typically reaching around 2 inches in length, is characterized by a prominent black wedge-shaped patch that extends from the tail to the midsection.

Above this patch, a brilliant coppery-orange to red hue graces the fish, creating a captivating contrast. The lower part of the body is silver, adding to its overall visual appeal.

Each Harlequin Rasbora has its own unique color shades, and the colors might change depending on the water conditions in the tank. It’s like each fish has its own special palette, and the environment can affect how vibrant or muted their colors appear.

Difference Between Males and Females

Males and females do look different. Males shine more and are skinnier. Their color is bright orange, like shiny metal, and they have a straight edge to their black patch near the top fin.

Females are bigger with rounder bodies that sparkle gold. They carry a rounded black wedge on their back side that pokes out a bit at the end.

Personality & Behavior

These are very peaceful fish that love to hang out with their friends and are happiest in groups. When you watch them, you’ll see they swim together in a way that looks like a dance. They don’t fight and get along well with other friendly fish in the tank.

These little swimmers enjoy exploring too. They will zip all around the plants and decorations in their home. But don’t worry, they won’t bother anyone else while doing it. They make great pets if you want a tank full of life but without any trouble!


With proper care, these fish can thrive for an impressive 5 to 8 years. Offering them the right food, maintaining clean water conditions, and ensuring a safe and comfortable habitat can transform them into your vibrant and long-lasting aquatic companions.

Harlequin Rasbora Care & Tank Set Up

Mastering their care is a journey into the art of aquarium creation, and with proper tank setup, you’re setting the stage for these vibrant fish to thrive.

Tank Size

These fish will need at least a 10-gallon tank to be happy. This gives them ample room to swim and engage in playful activities that are crucial for their well-being and overall happiness. 

It is essential to provide a spacious aquarium environment that allows these energetic fish to exhibit their natural behaviors, promoting a healthy and thriving community within the tank

How Many Can Be Kept Together

Harlequin Rasboras are shoaling fish. They exhibit a natural tendency to form groups and thrive when kept in numbers. It is recommended to keep them in groups of 8 – 10 individuals. This helps create a sense of security and encourages their natural schooling behavior, promoting a harmonious and visually appealing aquarium community. 

In a tank that holds 10 gallons of water, you can start with about 8 Harlequin Rasboras. If you have more space, you can add more. Each extra fish means adding more water, so for every new rasbora, think about adding 2 gallons.

Water Parameters

  • Water temperature: Between 76°F and 80°F (24°C – 26°C)
  • Water pH levels: Between 6.0 and 7.8
  • Water hardness: 2-15 dGH 

Understanding the water parameters is crucial for maintaining a healthy environment.

Water Temperature:

These fish need warm water to stay healthy. They like it when the water is between 76°F and 80°F (24°C – 26°C). This makes them comfortable and helps their bodies function properly.

Water pH Levels

These fish thrive when the pH level is between 6.0 and 7.8. If the pH goes up or down too much, it can make them sick or worse, so work hard to keep it just right.

Keeping their water stable means doing regular changes. This helps stop big shifts in pH that could stress out these little swimmers.

Water Hardness

These fish thrive in soft to moderately hard water with a recommended hardness level ranging between 2 to 15 dGH (degrees of General Hardness). 

Maintaining water within this range is essential for their overall well-being, reproductive success, and vibrant coloration. Regular monitoring and adjustments to water hardness contribute to a suitable and stable environment for these engaging freshwater fish.

Water Current

In their tank, they appreciate a gentle flow that mirrors the serene streams of their natural habitat. A filter or air pump can help create this gentle current. It’s important to set it up right so your fish feel at home and can swim happily.

Make sure the water moves smoothly through the whole tank. This helps keep things clean and gives your rasboras the movement they love without the current being too strong for them.

What To Put In The Tank

Creating the perfect underwater sanctuary goes beyond just water; it’s about crafting a miniature ecosystem that replicates their natural environment. Let’s explore how to select the right mix of substrate, decorations, and plants to keep these vibrant fish healthy and content in their aquarium home.


I like to use fine, dark substrate as it replicates their natural environment in the wild and helps them feel safe. You can choose sand, gravel, or peat moss for the bottom of your tank.

Make sure the layer of substrate is about 2-3 inches deep. This gives the plant roots room to grow strong.


Use driftwood and rocks to create hiding spots and break up lines of sight. Ensure that the decorations are smooth to prevent injury.

Avoid sharp or rough decor that could harm their delicate fins. Additionally, be cautious with brightly colored substrates or decorations, as they prefer subdued lighting and too much brightness may cause stress.

Ideal Plants

Consider using Java Moss, Java Fern, and Anubias Nana as they offer excellent hiding spots and shade, providing a favorable environment for your fish. These plants play a role in maintaining water quality by absorbing nitrates and releasing oxygen.

You might want to add floating plants like Amazon Frogbit, Hornwort, and Water Sprite to your tank too. These are helpful because they create a cover at the top of the water.


These rasboras prefer subdued lighting, replicating the dim conditions of their natural habitat. Use floating plants, tall vegetation, or dimmable aquarium lights to create shaded areas. 

Avoid intense or direct lighting, as it may cause stress to the fish. Aim for a lighting period of around 8 -10 hours a day to simulate a natural day-night cycle.


Use a reliable aquarium heater to ensure the water remains within their required temperature range, promoting their well-being and encouraging natural behaviors. 

Place the heater near areas with water circulation to facilitate even heat distribution throughout the entire tank. Choose a heater that suits your tank size and has the correct power level in watts.

Regularly monitor and adjust the heater settings as needed to maintain a consistent and comfortable environment for your fish.


Opt for a gentle and adjustable filter. A sponge filter or a hang-on-back filter with a low flow rate works well. Ensure that the filter doesn’t create strong currents, as these fish prefer calm waters. Regularly clean and maintain the filter to uphold water quality and create a healthy environment for your fish.

Harlequin Rasbora Tank Mates

Choosing the right companions is crucial for maintaining a harmonious aquarium. Some ideal tank mates include:

  • Other Small Tetras: Such as Neon Tetras, Ember Tetras, or Lambchop Rasboras.
  • Dwarf Corydoras: Small, peaceful bottom-dwelling fish like Pygmy Corydoras.
  • Small Gouramis: Examples are Sparkling Gouramis or Honey Gouramis.
  • Small Rasboras: Like Chili Rasboras or Harlequin’s close relative, the Lambchop Rasbora.
  • Shrimp and Snails: Peaceful invertebrates such as Cherry Shrimp or Nerite Snails.

Ensure compatibility in terms of size, temperament, and water parameter preferences when selecting tankmates

Tank Mates To Avoid

Avoid aggressive or fin-nipping species and steer clear of larger, predatory fish that may see the smaller rasboras as prey. Some species to avoid include:

  • Large Cichlids: Aggressive cichlids may intimidate or harass Harlequin Rasboras.
  • Large Barbs: Some barb species can be fin nippers, posing a threat to the delicate fins of Harlequin Rasboras.
  • Aggressive Bettas: Betta fish can be territorial and may not be suitable tank mates.
  • Large Catfish: Avoid large catfish that might view smaller fish as potential prey.

Harlequin Rasbora Food & Diet

Harlequin Rasboras are omnivores, meaning they consume both plant and animal matter. Their diet should be varied to meet their nutritional needs. Feed them a combination of:

  • High-Quality Flakes or Pellets: These should be the staple of their diet, providing essential nutrients.
  • Live or Frozen Foods: Offer occasional treats such as live or frozen brine shrimp, daphnia, or bloodworms to mimic their natural diet.
  • Vegetable-Based Foods: Supplement their diet with vegetable-based options like spirulina flakes or blanched vegetables.
  • Freeze-Dried Treats: Freeze-dried tubifex worms make for a tasty snack.

Feed in small amounts multiple times a day, replicating their natural feeding behavior. Ensure that the food is appropriately sized for their small mouths.

These little fish aren’t picky eaters, but a balanced diet contributes to their bright color and active behavior in my aquarium.

Breeding Harlequin Rasboras

Set up a separate tank with fine-leaved plants like java moss for egg attachment. You can also add Peat Moss to incorporate the humic acids found in blackwater environments.

Introduce a group of rasboras to the breeding tank. You’ll want a ratio of two females for every male.

Slowly increase the water temperature to the higher end of their range (around 78-79°F or 25-26°C). You need to do this slowly to avoid shock.

Provide them with nutrient-rich foods, including live or frozen foods, to enhance their reproductive readiness. Bloodworms and Daphnia are good options.

When they are ready, the female will select a nice leaf and signal the male to come over. She will release the eggs under the leaf, and the male will fertilize them. The eggs then float upward and stick to the underside of the leaf.

This happens multiple times while they’re breeding. Normally, they lay smaller batches of around 12 eggs at once. After the breeding process has finished, you could end up with about 300 eggs.

When breeding is completed, remove the adult fish as they will try to eat the fry. Approximately 24 hours post-fertilization, the fry will hatch from their eggs. During this time, they will remain on the leaf, consuming the nutrient-rich egg sac over the next 24 hours.

After hatching, feed the fry with infusoria or liquid fry food until they are large enough to consume brine shrimp or finely crushed flakes.

Remember, breeding fish requires patience and careful observation. If encountering challenges or seeking specific advice, consult with experienced aquarists or professionals in the field.

You’ll notice the fry will start swimming independently. Introduce finely-sized infusoria as their primary food source, maintaining this diet for approximately two weeks before transitioning to powdered fish food. These fish typically attain full maturity within a relatively short period, around six months.

Harlequin Rasbora Common Health Issues

Harlequin Rasboras can get sick like other fish. Clean water and good food keep them healthy. Here are common health problems they might face:

Ich is a disease that makes white spots on the fish’s body and fins. It comes from tiny parasites in the water.

Fin Rot happens when the edges of their fins start to look torn or rotten. Dirty water usually causes this.

Velvet disease gives fish a dusty, yellow-brown coat on their skin. It’s another parasite that loves bad water conditions.

Swim Bladder issues make it hard for fish to control floating or sinking. This can come from overfeeding or poor diet.

Dropsy is a symptom rather than a disease itself. It is often associated with an internal issue, such as kidney failure, bacterial infection, or problems with fluid balance. Your fish will display a swollen abdomen and raised scales are symptoms of this internal issue.

If you do notice any concerning symptoms, it is crucial to consult with a professional aquarium veterinarian for an accurate diagnosis before starting any treatment. Early detection and proper identification of diseases improve the chances of effective treatment and their overall well-being.

Are Harlequin Rasboras Right For You?

Consider Harlequin Rasboras if you prefer:

  • Peaceful Community: They thrive in peaceful community setups with other compatible fish.
  • Small Tank Size: Well-suited for smaller aquariums due to their modest size.
  • Vibrant Colors: Enjoy the striking colors and unique markings of Harlequin Rasboras.
  • Schooling Behavior: Appreciate fish that exhibit schooling behavior, creating an engaging aquarium dynamic.
  • Low-Maintenance: Seek low-maintenance fish that are relatively easy to care for.
  • Soft Water Preference: If your water conditions align with their preference for soft to moderately hard water.

Always research and ensure compatibility with your existing setup and personal preferences before introducing any new fish species.

Wrapping Up

Harlequin Rasboras are a delightful and low-maintenance fish for your freshwater aquarium. Their sparkling colors and lively energy make them a favorite in home aquariums.

Having these fish is just pure fun, they will effortlessly keep you entertained. Get ready to spend a lot of time just watching your tank!

I’m Elle, the founder of FishHQ. I created this website to share knowledge, tips, and inspiration for beginner hobbyists to help them create a healthy, happy, and vibrant environment for their fish to thrive. Read more...