Harlequin Rasbora – Trigonostigma heteromorpha – 2.5–3cm

The Harlequin Rasbora (Trigonostigma heteromorpha) is one of the most popular peaceful schooling fish in the freshwater aquarium hobby.

Known for its warm copper-orange body colour and distinctive black triangular marking, the Harlequin Rasbora is an excellent choice for planted aquariums, peaceful community tanks and aquascapes.

At approximately 2.5–3cm, these fish are still young and will continue to develop stronger colour and body shape as they mature.

They are best kept in groups, where their natural schooling behaviour creates constant movement and a much more impressive display.

At a Glance

Scientific Name: Trigonostigma heteromorpha
Common Name: Harlequin Rasbora
Current Size: Approximately 2.5–3cm
Adult Size: Approximately 4–5cm
Origin: Southeast Asia
Temperament: Peaceful
Care Level: Easy
Diet: Omnivore
Temperature: Approximately 23–28°C
pH: Approximately 5.0–7.5
Water Hardness: Soft to moderately soft
Minimum Tank Size: Approximately 60L recommended
Swimming Level: Mid to upper levels
Social Behaviour: Schooling
Breeding: Egg layer

Fish Profile

Harlequin Rasboras originate from Southeast Asia, where they inhabit slow-moving streams and forest waterways.

These environments are often shaded by overhanging vegetation and surrounded by leaf litter, branches and submerged roots.

In the aquarium, they are active but not frantic swimmers and are generally very peaceful toward other fish.

Their calm behaviour and relatively small adult size make them one of the easiest schooling species to incorporate into a community aquarium.

Colour & Appearance

Harlequin Rasboras are immediately recognisable by their bold black wedge-shaped marking that runs across the rear half of the body.

Their body colour usually ranges from silver-bronze to copper-orange, becoming stronger when the fish are healthy, settled and kept in good conditions.

Typical features include:

• copper to orange body colour
• distinctive black triangular marking
• silver highlights
• transparent to reddish fins
• streamlined body shape

Colour can vary between individuals and may intensify with maturity, diet, lighting and tank background.

Schooling Behaviour

Harlequin Rasboras are highly social fish and should be kept in groups.

A group of at least 6 fish is recommended, although 8–12 or more will usually produce a much more natural and attractive display.

When kept in a proper school, they generally show:

• stronger colour
• more confident behaviour
• reduced stress
• more natural movement
• less hiding

Larger groups also make their copper and black patterning much more visually impressive in planted aquariums.

Temperament & Behaviour

Harlequin Rasboras are peaceful and generally very easy to mix with other community fish.

They spend most of their time swimming through the middle and upper parts of the aquarium.

They are not aggressive and are unlikely to bother slower or smaller tank mates.

During breeding or courtship, males may display more actively toward females, but this behaviour is usually harmless.

Suitable Tank Mates

Harlequin Rasboras work well with a wide range of peaceful freshwater species.

Suitable companions may include:

• Tetras
• Corydoras
• Bristlenose Catfish
• small Gouramis
• peaceful Dwarf Cichlids
• other Rasboras
• small Loaches
• peaceful livebearers
• Otocinclus
• shrimp in suitable aquariums

Avoid:

• large predatory fish
• aggressive cichlids
• highly territorial species
• fish large enough to eat them
• persistent fin nippers

Their peaceful nature makes them especially suitable for community and planted aquariums.

Aquarium Setup

Harlequin Rasboras look especially good in planted tanks.

A suitable setup should include:

• live plants
• open swimming areas
• driftwood or root-style décor
• shaded areas
• gentle to moderate water movement
• reliable filtration
• a secure lid

Dark substrate and green plants can help make their copper-orange colour stand out.

Floating plants can also provide shaded areas and help create the softer lighting conditions these fish often appreciate.

Water Conditions

Harlequin Rasboras generally prefer warm, soft to moderately soft water.

A suitable range is:

Temperature: 23–28°C
pH: Approximately 5.0–7.5
Hardness: Soft to moderately soft

They are reasonably adaptable once settled, but stable water quality should always be prioritised over trying to constantly adjust the aquarium to an exact number.

Regular water changes and consistent maintenance will help maintain good colour, activity and health.

Feeding

Harlequin Rasboras are omnivorous and usually easy to feed.

Offer a varied diet including:

• quality tropical flakes
• small pellets
• micro granules
• frozen brine shrimp
• daphnia
• mosquito larvae
• suitably sized live foods

Because they have relatively small mouths, foods should be appropriately sized.

A varied diet helps maintain strong colour, body condition and breeding readiness.

Feed small amounts once or twice daily rather than allowing excess food to accumulate in the aquarium.

Growth & Adult Size

Fish supplied are approximately 2.5–3cm.

Adult Harlequin Rasboras generally reach around 4–5cm.

Their small adult size makes them suitable for modest aquariums, but a longer tank footprint is still useful because they are active schooling fish.

A tank of around 60L or larger gives a group enough room to school naturally while still allowing space for planting and décor.

Planted Aquarium Compatibility

Harlequin Rasboras are an excellent choice for planted aquariums and aquascapes.

They do not damage healthy plants and generally spend their time swimming through open areas rather than digging or rearranging substrate.

Their copper-orange colour provides a strong contrast against green plants, dark wood and darker substrate.

They work particularly well in aquascapes that include:

• Cryptocoryne
• Java Fern
• Anubias
• Bucephalandra
• Mosses
• Vallisneria
• floating plants

They are also a popular choice for nature-style aquariums and heavily planted community tanks.

Breeding

Harlequin Rasboras are egg-laying fish.

Unlike many Rasboras that scatter eggs freely, Harlequins often deposit adhesive eggs on the underside of broad plant leaves.

A breeding pair may repeatedly turn beneath leaves during spawning.

The adults do not provide parental care and may eat the eggs, so a separate breeding tank is usually recommended if raising fry is the goal.

Soft, slightly acidic water and broad-leaved plants can help encourage spawning.

Once hatched, the fry are extremely small and initially require very fine foods before progressing to larger foods such as freshly hatched brine shrimp.

Is the Harlequin Rasbora Right for You?

Harlequin Rasboras are a great option for aquarists wanting:

• a peaceful schooling fish
• strong colour without aggressive behaviour
• a planted aquarium species
• an easy community fish
• a beginner-friendly tropical species

They are especially effective when kept in larger groups, where their schooling behaviour and contrasting colour become much more noticeable.

Their combination of hardiness, peaceful temperament and attractive appearance makes them one of the most reliable choices for freshwater community aquariums.

Quick Care Guide

Current Size: Approximately 2.5–3cm
Adult Size: Approximately 4–5cm
Tank Size: 60L+ recommended
Group Size: 6+ fish, with larger groups preferred
Temperature: 23–28°C
pH: 5.0–7.5
Temperament: Peaceful
Diet: Omnivore
Swimming Level: Mid to upper levels
Breeding: Egg layer
Difficulty: Easy

Important Information

Fish supplied are approximately 2.5–3cm unless otherwise stated.

Harlequin Rasboras are schooling fish and should be kept in groups rather than individually.

Individual fish may vary naturally in body colour, markings, size and shape.

Actual fish received may differ slightly from photographs due to age, lighting, condition and natural individual variation.