Harlequin rasbora

Water parameters

Tolerated range for this species. Aim for the middle of each band rather than the extremes.

Tank and habitat

Substrate: any.

Behavior

Plant interaction: plant safe.

Feeding

Undemanding. Accepts flake, micro pellets, frozen daphnia, brine shrimp, and bloodworm. Not a picky feeder. Two small feedings per day are better than one large one. Harlequins feed in the midwater column and rarely go to the bottom for scraps, so pair with corys or loaches if you want the bottom cleaned.

Compatibility

• One of the safest community fish available. Peaceful with everything its own size or larger.
• Works well with tetras, corys, small gouramis, rasboras, and peaceful dwarf cichlids like rams and apistos.
• Large enough that most common community fish won't eat them, unlike smaller rasboras (chili, phoenix).
• Avoid large or aggressive tankmates. Not a target for fin nippers because the fins are short.
• Safe with shrimp. Adult harlequins ignore cherry shrimp and amano shrimp.

Origin and habitat

Native to the Malay Peninsula (Perak, Selangor, Negri Sembilan, Johor, Pahang), Singapore, Sumatra, and the Riau archipelago (Tan 2020 paper). Also reported from southern Thailand. Found in primary peat swamps in groups larger than 100. First wild fish imported to Europe in 1906 (Tetra blog). Described by Duncker in 1904 as Rasbora heteromorpha. Moved to Trigonostigma by Kottelat and Witte in 1999. The genus was distinguished from Rasbora by small adult size (under 35mm SL), the broad black triangular body marking, incomplete lateral line (6-9 pored scales), and unique spawning behavior: upside-down body positions of both male and female, with egg deposition on the underside of leaves (Tan 2020, Kottelat and Witte 1999). The spawning itself: the female rubs her belly against a leaf underside, the male swims upside-down alongside and clasps her, fertilizing 6-12 eggs at a time with a total of about 100-300 eggs per spawn. Five species in the genus: T. heteromorpha, T. espei, T. hengeli, T. somphongsi, and T. truncata. T. somphongsi is extremely rare and possibly extinct in the wild. Size 5 cm. It is the best known and most widely kept species among the rasboras.

Breeding

Unusual among small fish: harlequins deposit eggs on the underside of broad leaves (anubias and cryptocoryne work well). The female flips upside-down and presses eggs against the leaf underside, where the male fertilizes them. Breeding requires very soft acidic water (pH 5.5-6.0, GH below 3) and dim lighting. Condition with frozen food for two weeks before attempting. Most hobbyists never breed them because the water parameters are demanding, but it's been done. Eggs hatch in about 24 hours. Fry need infusoria for the first few days, then baby brine shrimp.

Common problems

Hardy fish with few species-specific issues. Ich is the most common disease, usually from transport stress in newly purchased fish. Harlequins sometimes lose color intensity when stressed or in poor water; a washed-out orange is a sign something is wrong (high nitrate, unstable temperature, or aggression from tankmates). Occasional fungal infections on the body, treated with standard antifungal medication. Lifespan is 5-6 years with good care; fish dying at 1-2 years old is a sign of chronic poor conditions, not normal aging.

Bioload

Further reading

• Stocking ideas by tank size