Harlequin Rasbora Care Guide

What are harlequin rasboras?

Harlequin rasboras (Trigonostigma heteromorpha) are small, peaceful schooling fish from Southeast Asia. They display copper-orange bodies with a distinctive black triangular patch behind the gills — the “harlequin” marking that gives them their name. They grow to about 1.5–2 inches and typically live 4–6 years in good conditions. They thrive in planted tanks with gentle flow and soft, slightly acidic water.

Tank size and setup

A 10-gallon tank can hold a small group, but 20 gallons or more is better for a proper school of eight to ten fish. Harlequins are mid-water swimmers and appreciate open space plus dense planting along the sides and back. Dark substrate and driftwood or Indian almond leaves mimic their natural blackwater habitat and can bring out their colour. Use gentle filtration; strong current stresses them.

Water parameters

Keep temperature at 72–80 °F (22–27 °C). pH around 6.0–7.5 suits them; soft to moderately hard water (2–15 dGH) works well. Ammonia and nitrite must stay at zero; keep nitrate low with regular water changes. They come from slow, tannin-stained streams, so slight tint from driftwood or leaves is natural and often appreciated. Track water parameters with a test kit or app like App-aquatic.

Feeding

Harlequin rasboras are omnivores. Offer quality tropical flake or micro-pellets as a base, plus occasional frozen or live foods such as brine shrimp, daphnia, or bloodworms. Small, frequent feeds work better than one large meal. Avoid overfeeding; uneaten food degrades water quality.

Rasbora types: harlequin vs chili and others

Several rasboras are popular in the hobby. Chili rasboras (Boraras brigittae) are much smaller — under an inch — and more delicate. They prefer very soft, acidic water and suit nano tanks (5–10 gallons). They are bright red with a dark stripe and need a mature, stable setup. Lambchop rasboras (Trigonostigma espei) look similar to harlequins but with a smaller, more rounded black patch. Scissortail rasboras are larger and more active. Harlequins are the most forgiving of the group and a solid choice for beginners.

Tank mates

Harlequin rasboras are peaceful and suit community tanks. Good companions include neon or cardinal tetras, corydoras, otocinclus, dwarf or honey gouramis, and other small, non-aggressive species. Avoid fin-nippers (e.g. tiger barbs) and large or predatory fish. Keep harlequins in a group of at least six; eight to ten is better for confident schooling behaviour. See betta tank mates — harlequins often work with bettas in a 15+ gallon tank.

Common illnesses

Harlequin rasboras can be affected by the same conditions as other tropical fish. Ich (white spot) appears as small white dots; raise temperature slightly and treat with an ich remedy. Fin rot shows as frayed or receding fins — improve water quality and consider a broad-spectrum treatment. Bacterial infections can cause redness, ulcers, or lethargy; isolate affected fish and treat in a hospital tank. Prevention is best: quarantine new fish, maintain stable parameters, and avoid overcrowding. Stress from poor water or aggressive tank mates weakens their immune system.

Quick takeaways

20 gallons or more for a school of 8–10; planted tank with gentle flow.
72–80 °F, pH 6–7.5, soft water; tannins from wood or leaves are fine.
Chili rasboras are smaller and more delicate; harlequins are the hardier choice.
Peaceful tank mates: tetras, corydoras, gouramis; avoid fin-nippers.
Watch for ich, fin rot, and bacterial infections; good water quality is the best prevention.