Harlequin rasbora

What it is

This species belongs to the carp family and displays a lozenge-shaped body. Its base colour ranges from orange to pink, shifting with water conditions. A large, roughly triangular black marking covers the rear half of the body, starting below the dorsal fin. The fins carry red tints, with the forked tail showing concentrated colour on the outer rays. Males typically have a larger black patch with a rounded section near the anal fin, while ripe females appear fuller. Adults reach about four to five centimetres in length.

Where it lives

Native waters include the Malay Peninsula, Singapore, Sumatra, and Borneo. These fish inhabit streams flowing through peat swamp forests. The soil inhibits leaf decay, creating peat that leaches humic acids into the water. This results in low mineral content and chemistry similar to South American blackwater habitats like the Rio Negro. Populations once thought to be this species in southern Thailand were identified as the related Trigonostigma truncata in 2020.

How it lives

Harlequin rasboras are shoaling fish that thrive in groups of six or more. They are peaceful and suitable for community tanks with other small, calm species like characins, barbs, danios, and Corydoras catfishes. They prefer planted aquaria with moderate filtration and subdued lighting. In their natural habitat, they navigate slow-moving waters. They accept various prepared foods but benefit from live options like Daphnia or mosquito larvae for optimal health.

Breeding and life cycle

Unlike many rasboras, this species attaches eggs to the undersides of plant leaves rather than scattering them. Spawning occurs in soft, acidic water at around twenty-eight degrees Celsius. Pairs adopt an inverted position beneath leaves during reproduction. A female may lay up to three hundred eggs over two hours. Parents are usually removed after spawning to prevent egg consumption. Fry hatch in eighteen hours and require fine food like infusoria before transitioning to brine shrimp.

Fish and people

Introduced in the early 1900s, this fish quickly became a favourite among aquarists. It appeared on the cover of William T. Innes's classic book Exotic Aquarium Fishes in 1935, stamped in gold. The image remained through all nineteen editions. Aquarists value it for its active disposition and the visual impact of large shoals. It is easily maintained in home setups provided water cleanliness is prioritised, though breeding demands specific chemical parameters.

Status and threats

The species is widespread in its native range and commonly bred in captivity. No specific threats or conservation concerns are detailed in the provided material. Its adaptability to varying aquarium conditions, within limits, supports its popularity. The main challenges for keepers involve replicating soft, acidic water chemistry for successful breeding attempts.

Relatives and look-alikes

It is often confused with Trigonostigma espei and Trigonostigma hengeli, which were previously considered subspecies. These relatives have more slender bodies and distinct markings. Instead of a triangle, they feature a horizontal stripe thickened below the dorsal fin, resembling a lamb chop. This resemblance has led to the common name 'lamb chop rasboras' for those similar species.

Questions people ask about Harlequin rasbora

Good reading and sources

• Harlequin rasbora on WikipediaEncyclopedia
• More photos on Wikimedia CommonsPhotos
• Wikidata recordOpen data