Harlequin Rasbora

The Harlequin Rasbora reaches only about 5 cm, yet displays striking colouration that ranges from pale pink to bright orange with a copper sheen on the upper and lower edges of the body. A distinctive feature is a black triangular patch tapering towards the tail, sometimes with a blue or violet tint. Selectively bred forms have different colours: the Blue Harlequin has a blue body, while the Black Harlequin's body matches the colour of the triangular patch. Males and females differ: males are usually slightly smaller, and their black patch is more rounded with elongated points at the edges.

The red-striped rasbora was first described in 1904, though earlier references probably concerned other species. Its natural range spans Southeast Asia, both mainland and islands. In the wild, it prefers shaded forest streams and channels, as well as peat swamps, where water flow is absent or very weak. The water in such places often turns yellowish or brown from decomposing organic matter such as leaves, grass and fruit.

The Harlequin Rasbora gets along well with most peaceful aquarium fish of similar size. Keep them in groups of at least 10 individuals. Under these conditions, males display brighter colours as they compete for the females' attention. A smaller group (fewer than 10) may show some restlessness towards other tank inhabitants, trying to involve them in their activities—not all fish species tolerate this.

To keep these fish successfully, the water must be slightly acidic, which can be achieved using a filter with appropriate material. The filter's flow rate should be chosen with the tank size in mind, as this fish does not tolerate strong currents. Perform a weekly partial water change of at least one quarter of the tank volume. Dimmed lighting that mimics conditions under a forest canopy is preferable. The tank looks best with rooted plants with large leaves that reach the surface; dense growth should run along the sides, and floating plants provide extra shade. Use a dark substrate, decorated with driftwood or bogwood pieces.

This omnivore feeds on small insects, worms, crustaceans and zooplankton in nature. In the aquarium, supplement dry food (flakes or granules) with live foods such as bloodworm at least twice a week, or use dried bloodworm. Plant-based foods such as finely chopped lettuce or spinach also make a useful dietary addition.

Breeding this species requires careful planning and a separate tank. To trigger spawning, feed high-protein foods daily for a month. Prepare a separate aquarium of at least 40 litres, filled to no more than 20 cm depth. Optimal water parameters: very soft (1–3 dGH), slightly acidic (pH 5.3–6.0) and temperature 25–28°C. A simple sponge filter running at low power is sufficient, and dimmed room light works. Live or artificial broad-leaved plants for egg attachment are essential. Use coarse pebbles as substrate. When males become brightly coloured and the female's belly is full, move the pair to the prepared tank and continue feeding high-protein foods. Spawning may begin within days. The male performs a courtship dance, circling the female and pointing to the chosen plant where eggs will be laid. Eggs attach to the underside of the leaf; fallen eggs roll under pebbles and become inaccessible to the parents, who readily eat their own eggs. After spawning, return the adults to the main tank. Lower the water level in the breeding tank to 15 cm, ensuring the eggs remain submerged. Fry appear after one day and begin free-swimming in 3–5 days.

When kept in favourable conditions, disease is rare. Poor water quality or introducing infected fish usually causes problems.

When selecting Harlequin Rasboras, observe the activity of fish in the group: healthy individuals should be mobile, brightly coloured and show obvious schooling behaviour. Avoid fish showing signs of emaciation, torn fins, or visible spots on the body, as these may indicate stress or disease. Buy a group of 8–10 individuals at once; solitary keeping or very small groups make the fish timid and chronically stressed.