Harlequin Rasbora

Habitat

Natural Habitat: Shaded forest streams and slow-flowing tributaries with dense marginal vegetation, overhanging canopy, submerged roots, branches, leaf litter, and soft substrates.

Natural Water Conditions: Warm, soft, acidic to near-neutral water, often stained amber by tannins and low in dissolved minerals. Seasonal rainfall changes depth, flow, and chemistry.

Other Habitat Notes: Filtered forest light and dark leaf-littered bottoms help the fish feel secure and intensify their copper-orange coloration and black flank markings.

Aquarium

Tank Size Notes: Horizontal swimming length matters. A longer aquarium permits cohesive schooling, calmer social interactions, and enough open water for a larger group.

Tank Shape: A long, standard rectangular aquarium

Substrate: Dark sand or smooth fine gravel is ideal for appearance and natural presentation, though any inert, non-sharp substrate compatible with the plants can work.

Plants: Use dense planting along the back and sides with open swimming room. Cryptocoryne, Java fern, Anubias, mosses, broad-leaved plants, and floating plants are suitable.

Hardscape: Branching driftwood, smooth roots, rounded stones, and optional aquarium-safe leaf litter

Hiding Places: Create shaded planted edges and retreats without breaking up all of the open midwater schooling space.

Swimming Space: Provide a broad, unobstructed central or front area for the school to cruise together.

Water Flow: Gentle to moderate with good oxygenation; avoid turbulent flow across the entire tank.

Lighting: Low to moderate. Floating plants and shaded areas produce a natural look and reduce nervous behavior.

Lid Required: Yes. Harlequin rasboras can jump when startled or chased.

Other Setup Notes: Use a fully cycled aquarium with ammonia and nitrite at 0 ppm. Keep the group away from exposed, high-traffic locations and use a background, plants, and subdued lighting to help newly introduced fish settle.

Behavior

General Behavior: An active but not frantic midwater school. The fish cruise together, pause beneath cover, investigate food quickly, and may display by swimming beside one another with fins extended.

Aggression: Very low; harmless chasing or display may occur within the group.

Territorial: No

Schooling/Social Behavior: Forms a loose-to-cohesive school that tightens when startled. A larger group generally produces greater confidence, better color, and more natural movement.

Group Requirements: Keep at least 8 of the same species. Similar Trigonostigma species may school nearby, but they do not replace an adequate same-species group.

Notable Behaviors: During courtship, a female may turn upside down beneath a broad leaf while the male wraps around her and eggs are attached to the underside.