Harlequin rasbora

Quick facts

Adult size

4.5 cm

Lifespan

can live up to 6 years; captive average is 4-5

Tank zone

mid

Temperament

peaceful

Difficulty

beginner

Schooling

recommended 6+ (critical minimum 4, thrives at 10+)

Water parameters

Temperature

22–27°C

pH

5.0 to 7.5

Hardness

1 to 12 dGH

Tank requirements

Minimum volume

75 L

Minimum length

60 cm

Flow

low

Lighting

dim preferred

Substrate

any

Driftwood

preferred

Feeding

Diet: omnivore, feeds primarily at the mid.

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.

Habitat

Native to slow-moving, heavily shaded streams and peat swamp forests in peninsular Malaysia, Singapore, Sumatra, and Borneo. Wild habitats are tannin-stained blackwater with pH as low as 4.0 and negligible hardness. Tank-bred stock (virtually all fish in the trade) adapts to a much wider range, thriving at pH 6.0-7.5. The distinctive black triangular patch on the rear half of the body is the most recognizable feature. One of the oldest fish in the hobby, first imported to Europe in 1906. Still among the top 10 best-selling tropical freshwater fish worldwide because nothing else fills the same niche: a small, hardy, brightly colored schooling fish that works in almost any community.

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

Bioload coefficient: 1.2 (deep-bodied 4.5 cm species with mellow swimming activity; pulled down slightly from formula).

Bioload coefficients are calibrated against the neon tetra as the anchor (1.0). See the methodology page for the formula and how each value was derived.

Further reading

• Stocking ideas by tank size