Introduction The Harlequin Rasbora (Trigonostigma heteromorpha) is one of the most iconic and widely kept freshwater aquarium fish in the hobby. Originating from the peat swamp forests and slow-moving streams of Southeast Asia — particularly Thailand, Malaysia, Singapore, and Sumatra — these small, active schooling fish are prized for their striking copper-orange coloration and distinctive black triangular patch. Hardy, peaceful, and visually stunning in groups, they're an excellent choice for community aquariums of all experience levels. Species Overview Scientific Name: Trigonostigma heteromorpha Common Names: Harlequin Rasbora Family: Cyprinidae Origin: Southeast Asia (Thailand, Malaysia, Singapore, Sumatra) Size: 1.5–2 inches (3.8–5 cm) Lifespan: 5–8 years with proper care Temperament: Peaceful, active schooling fish Care Level: Easy Aquarium Setup Tank Size A minimum of 20 gallons or more is strongly recommended to keep a proper group of 8–12 fish and allow them to exhibit natural schooling behavior. Harlequin Rasboras are active mid-water swimmers and appreciate horizontal swimming space, so longer tanks are preferred over tall ones. Water Parameters Harlequin Rasboras originate from soft, acidic blackwater environments. While tank-bred specimens are more adaptable, they thrive best in conditions that reflect their natural habitat: Temperature: 72–81°F (22–27°C) pH: 5.5–7.5 (optimal: 6.0–7.0) Hardness: 1–12 dGH (soft to moderately hard) Ammonia / Nitrite: 0 ppm Nitrate: <20 ppm Perform weekly water changes of 25–30%. Avoid sudden parameter swings — Harlequins are sensitive to rapid changes in water chemistry. Filtration and Flow Use a quality hang-on-back (HOB), sponge filter, or canister filter rated for your tank volume. Harlequin Rasboras prefer gentle to moderate flow — strong currents can stress them. Use a spray bar, sponge pre-filter, or baffle to diffuse output. Sponge filters work well in smaller setups and are safe for fry. Substrate and Décor A planted aquarium suits Harlequin Rasboras perfectly. Aim for: Dark substrate — fine dark sand or dark gravel enhances their copper coloration Dense planting — Java fern, Cryptocoryne, Anubias, and stem plants along the back and sides Open mid-water swimming space in the center of the tank Floating plants (Amazon frogbit, Salvinia) to diffuse light and create a more natural feel Driftwood and leaf litter — Indian almond leaves release tannins that mimic their blackwater habitat and have mild antibacterial properties Lighting Moderate to low lighting is preferred. Bright, harsh lighting can cause stress and cause the school to hide. Floating plants or a dimmer are effective solutions. Diet and Feeding Harlequin Rasboras are omnivores and unfussy eaters. In the wild they consume small insects, zooplankton, and plant matter. In the aquarium: High-quality micro pellets or flake food — the daily staple (choose small-sized pellets appropriate for their mouth size) Frozen/live foods — baby brine shrimp, daphnia, micro worms, and bloodworms for variety and conditioning Freeze-dried foods — acceptable as occasional treats Feed small amounts 1–2 times daily, offering only what they consume in 2–3 minutes. Their small mouths require appropriately sized food — oversized pellets will be ignored. Compatible Tankmates Harlequin Rasboras are ideal community fish. Their peaceful temperament and mid-water preference make them compatible with a wide range of species. Avoid any fish large enough to eat them or aggressive enough to cause stress. Excellent Tankmates Corydoras catfish — Bronze Cory, Panda Cory, Sterbai Cory; peaceful bottom-dwellers that occupy a different zone Small tetras — Neon Tetra, Cardinal Tetra, Ember Tetra, Rummy Nose Tetra Other rasboras — Chili Rasbora, Lambchop Rasbora, Espei Rasbora Dwarf Gourami Otocinclus — excellent algae-eating companions Peaceful Plecos — Bristlenose Pleco, Clown Pleco Livebearers — Guppies, Swordtails, Platies Dwarf cichlids — Apistogramma species in larger tanks with plenty of cover Shrimp and snails — Amano shrimp, Cherry shrimp, Nerite snails, Mystery snails Bettas — Some bettas coexist peacefully, others will harass or attack rasboras. Not recommended unless the betta's temperament is well-established Tankmates to Avoid Aggressive or nippy fish — Tiger Barbs, Serpae Tetras, larger barbs Large predatory fish — Cichlids (most species), Oscars, Arowanas, large catfish Behavior and Social Needs Harlequin Rasboras are obligate schooling fish and must be kept in groups of 8 or more — 10–12 is ideal. Smaller groups result in chronic stress, faded coloration, and hiding behavior. In a proper school you'll observe: Tight, synchronized schooling through open water Vibrant copper-orange coloration that intensifies in well-maintained conditions Active mid-water foraging and exploration Spawning displays — males will court females by swimming alongside and beneath them near broad-leafed plants Health and Common Issues Ich (White Spot Disease) — treat with heat (raise to 82°F/28°C) and a half-dose of ich medication; Harlequins are moderately sensitive to medications Velvet (Oodinium) — gold-dust appearance on the body; treat with copper-based medication (remove invertebrates first) Fin rot — usually caused by poor water quality; improve maintenance and treat with antibacterial medication if needed Neon Tetra Disease — rare but possible; no cure, remove affected fish promptly to prevent spread Faded coloration — often a sign of stress, poor diet, or suboptimal water parameters; address root cause first Conclusion Thank you for reading this article on Harlequin Rasboras. 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