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.

Compatibility

Good Tank Mates: Peaceful tetras, other small rasboras, pencilfish, small peaceful danios, kuhli loaches, Corydoras with matching temperature needs, Otocinclus, peaceful gouramis, dwarf cichlids in spacious planted tanks, adult shrimp, and snails.

Use Caution With: Very active or boisterous fish, fin-nippers, territorial cichlids, large gouramis, fish requiring very hard alkaline water, and predators small enough to appear peaceful but large enough to swallow them.

Avoid: Large predatory fish, aggressive cichlids, persistent fin-nippers, and tank mates that consistently outcompete the school for food.

Compatibility Notes: Their calm activity suits planted community tanks. Adult shrimp are usually safe, but eggs, shrimplets, and very small fry may be eaten.

Feeding

Captive Diet: A varied omnivorous diet based on quality small pellets or flakes plus frozen or live foods.

Recommended Foods: Micro pellets, finely crushed quality flakes, frozen or live daphnia, baby brine shrimp, cyclops, mosquito larvae from a safe source, and chopped or small bloodworms.

Feeding Frequency: Feed 1-2 small meals daily, offering only what the group consumes promptly.

Feeding Behavior: Feeds readily in the upper and middle water column. Use small particles and distribute food across the surface so shy or lower-ranking fish receive enough.

Special Requirements: Provide dietary variety and confirm every fish maintains a full body. Avoid oversized foods and chronic overfeeding.

Health

Signs of Stress: Clamped fins, faded color, rapid breathing, refusal to eat, persistent hiding, isolating from the group, erratic swimming, rubbing, white spots, sores, swelling, or loss of balance.

Common Keeper Mistakes: Keeping too few, using a very small or exposed tank, adding them before cycling is complete, making abrupt parameter changes, providing no plant cover, using strong flow everywhere, and housing them with aggressive or oversized fish.

Initial Checks: Check whether a withdrawn individual obtains suitably small food and whether it is being chased away from the group. Record measured water conditions, recent additions and when signs began; obtain qualified fish-health advice for ongoing appetite loss or abnormal breathing.

Suitability

Important Things Before Buying: Prepare a cycled 20-gallon-long or larger planted aquarium and buy at least 8. Select fish with full bodies, clear eyes, open fins, even breathing, intact scales, and normal schooling behavior. Avoid a sales tank with white spots, sores, severe thinness, clamped fins, or repeated unexplained deaths, and quarantine when possible.