Care guide · Freshwater · fish

Harlequin rasbora

Trigonostigma heteromorpha

Photo: Juan R. Lascorz · Source · CC BY-SA 3.0Resized and compressed; no cropping or colour changes.

The harlequin rasbora combines a copper-pink body with a broad black triangle on the rear flank. In a generous group these lively fish explore midwater without remaining in a permanently tight school. Trigonostigma heteromorpha comes from Southeast Asia and benefits from sheltered planted margins alongside clear swimming lanes. It is usually an approachable aquarium species when its group is large enough and the aquarium is mature. However, it grows larger and feeds more assertively than an ember tetra. That difference matters just as much as overlapping water requirements when choosing companions. Plan at least ten individuals, subdued lighting and sufficient tank length. Its unusual habit of laying eggs beneath leaves adds interest, but sound everyday care should come before any attempt to breed it.

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At a glance

Temperature: 23–26 °C
pH: 6.0–7.0
GH: 5–10 °dGH
Aquarium length: At least 60 cm
Minimum aquarium volume: At least 54 L
Adult size: Approximately 4–5 cm
Group size: At least 10 individuals
Swimming zone: Middle
Temperament: Peaceful
Lighting preference: Subdued lighting
Cover: Plants and sheltered areas
Staple food: Small granules, Fine flakes
Supplementary food: Live food, Frozen food, Finely crumbled dried natural food

Use 60 cm length and approximately 54 litres gross as a lower limit for a dedicated group. Prefer 80 cm and around 100 litres, particularly with companions.

About this species

Expect adults to reach roughly 4–5 cm. The broad dark wedge separates this fish from slimmer relatives such as Trigonostigma espei and T. hengeli. T. truncata also resembles it, so a common name alone is not a complete identification. Choose recognisable, healthy fish from a reliable group.

Females tend to be fuller and males slimmer, but colour and wedge shape do not always establish sex with certainty. Modern classifications place the fish in Danionidae; older accounts often use Cyprinidae. Ornamental colour forms still need the same fundamentals: companions of their own species, swimming space and suitable water. A different colour is not evidence of different social requirements.

Natural habitat

The known range includes the Malay Peninsula, Singapore and Bintan. Sheltered forest waters offer gentle flow, shade and structure along the margins. Organic material may stain the water, but this does not mean every aquarium population needs extremely acidic blackwater. Related fishes from the same region must not automatically be treated as this species.

Translate the useful habitat features into a dark background, planted edges, roots and open water between them. Gentle surface movement maintains gas exchange. Leaf litter is optional and requires controlled maintenance rather than replacing it. The linked plants provide functional shelter; their inclusion does not claim that they all grow together at one natural collection site.

Aquarium and layout

Use 60 cm length and approximately 54 litres gross as a lower limit for a dedicated group. Prefer 80 cm and around 100 litres, particularly with companions. Preserve a clear swimming lane: a densely packed tank cannot compensate for missing space through its nominal volume. Fine substrate, wood and robust leaves provide cover.

Soften lighting with plants without sealing the surface. A mature filter with moderate output is preferable to forceful flow throughout the aquarium. Fit a lid, stock only after ammonia and nitrite are undetectable, and feed cautiously at first.

Water conditions

Our routine maintenance range is 23–26 °C, pH 6–7 and approximately 5–10 °dGH. Specialist sources list broader limits, but their extremes are not everyday targets. Wild-caught fish or breeding projects may need softer water and separate assessment.

A specific KH minimum is not sufficiently established to prescribe here. With soft water, monitor stability and match replacement water carefully. Do not maintain unusually high temperatures simply to make a pairing with a warm-water cichlid appear suitable.

Behaviour and group size

At least ten allow social choice and distribute attention. Brief displaying or chasing can occur, but a persistently excluded fish needs attention. Harlequins are generally peaceful and plant-safe, although tiny shrimplets may be eaten.

Their efficient feeding requires care alongside ember tetras or pygmy corys: distribute food and watch the smaller fishes. A full-grown harlequin is less vulnerable than a tiny tetra, but is not automatically safe with a large cichlid. Each compatibility card assesses a separate pairing rather than proposing one combined community.

Feeding

A complete small granule or appropriately crushed flake can form the staple diet. Choose particles swallowed readily rather than repeatedly spat out. Offer small quantities once or twice daily and watch less confident individuals. Small live or thawed invertebrates provide variety.

Dried Daphnia is supplementary: break down oversized pieces and do not use it as the entire diet. These fish feed readily in midwater and at the surface. Bottom dwellers therefore need their own targeted ration rather than reliance on accidental leftovers.

Breeding

The pair turns beneath broad leaves to attach adhesive eggs to their undersides. There is no dependable parental care and adults may eat the eggs. Deliberate breeding normally uses a separate quiet aquarium, suitable leaves and soft, mildly acidic water. Breeding conditions are not mandatory everyday maintenance targets.

Remove the parents after spawning if raising fry. First foods must be very small, with suitable nauplii introduced later. Protect eggs and larvae from strong currents and filter intakes while maintaining water quality without abrupt changes.