Harlequin Rasbora

About the species

The harlequin rasbora is one of the few shoaling fish that can be recommended to complete beginners without reservation – unlike many other small aquarium fish, it tolerates a fairly wide range of water parameters and doesn't need carefully balanced soft water. It's recognisable by the distinctive black wedge or triangular patch on the rear half of its body, which also gave the genus its scientific name (Trigonostigma roughly means triangle mark). It comes from the peat swamps and slow-moving waters of Southeast Asia, and in the aquarium it's one of the most dependable shoaling fish for a mixed community tank.

Where the name and the black patch come from

The wedge-shaped patch is shaped slightly differently on every individual, but always as a triangle or wedge that widens toward the tail – it's what gave the species both its common and scientific names. The patch also serves a practical purpose: it helps identify individual fish within a shoal, and it's one of the features used to tell the harlequin rasbora apart from related species such as the lambchop rasbora, which carries a similar but differently shaped patch.

The peat swamps of Southeast Asia

The harlequin rasbora comes from the Malay Peninsula, southern Thailand, Singapore, and the islands of Sumatra and Borneo, where it inhabits slow streams and peat swamps with soft, slightly acidic water. Despite that origin, the species is surprisingly adaptable to a wider range of water parameters in captivity, setting it apart from many other shoaling fish from the same region.

Why it tolerates more than many other shoaling fish

The harlequin rasbora tolerates temperatures of roughly 22 to 28°C, pH of 5.0 to 7.0, and general hardness of 5 to 12°dH — a considerably wider range than many other small shoaling fish from similar habitats. That adaptability is exactly why it's one of the few shoaling fish that can safely be recommended to an aquarist with only moderately soft tap water, not just one with specially prepared soft water.

Eggs on the underside of a leaf

Unlike most small shoaling fish, which simply scatter their eggs among plants, the harlequin rasbora lays its eggs on the underside of broad leaves, such as Java fern, with the female turning upside down to attach them. This behaviour is unusual among small aquarium fish, which is why experienced breeders often pick the harlequin rasbora as a first attempt at breeding a shoaling species.

Shoaling and behaviour

A group of at least 8 to 10 is recommended, in which the harlequin rasbora shows its characteristic tight, cohesive swimming through the middle of the tank. The species is markedly peaceful and shows no aggression toward its own kind or other fish, which is why it also works well as a so-called dither shoal — a group whose relaxed behaviour reassures more timid tankmates, such as dwarf cichlids, which tend to leave hiding spots sooner in the presence of a calm shoal.

Living together in the tank

The harlequin rasbora gets along well with most peaceful species of similar size — other tetras, corydoras catfish, otocinclus, and dwarf cichlids. Since it's a calm species with no territorial behaviour, avoid aggressive or fast-swimming species that would push it into a corner of the tank.

A tank that mirrors the peat swamp

Although the harlequin rasbora tolerates a wide range of water parameters, it looks its best in a densely planted tank with subdued, slightly tinted lighting that mirrors its natural peat-swamp habitat. Floating plants that partially shade the light, together with a dark substrate, further bring out its orange-red colouring and reduce stress in more timid individuals within the shoal.

Diet

In the wild it hunts small insects, worms, and crustaceans; in the aquarium it accepts quality small-grain pellets and flakes, supplemented with frozen or live foods such as brine shrimp and daphnia. Regular but moderate feeding once or twice a day is sufficient.

Common mistakes

The most common mistake with the harlequin rasbora isn't about water chemistry but about too small a shoal — in a group of fewer than five, the species becomes skittish and doesn't show its characteristic tight swimming. A second common mistake is housing it with much larger or aggressive species, which, despite its hardiness, stresses it over the long term.

Pros & cons