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

Trigonostigma heteromorpha is the harlequin rasbora, the deepest-bodied and best known of the small Trigonostigma rasboras and one of the most widely kept aquarium fish of all. It comes from the Malay Peninsula, Singapore, Sumatra, and nearby islands, where it lives in soft, acidic, tannin-stained peat swamps and shaded forest streams, often in large groups. Duncker described it in 1904 as Rasbora heteromorpha, and Kottelat and Witte moved it into Trigonostigma in 1999. That genus was set apart from Rasbora by a small adult size, an incomplete lateral line, the bold black triangular mark on the rear half of the body, and a distinctive way of spawning on the undersides of leaves. The harlequin has the broadest, darkest triangle of the group over a pinkish body, which separates it from the slimmer-marked T. espei and the smaller, paler T. hengeli. It reaches about 4.5 cm, with a maximum near 5 cm. The genus holds five species in all, including the rare T. somphongsi, which may be extinct in the wild.

Breeding

Harlequins have an unusual spawning style for a small cyprinid. Rather than scattering eggs, the female turns upside down beneath a broad leaf, an anubias or cryptocoryne works well, and rubs her belly against the underside while the male clasps her and fertilises the eggs against the leaf. She places a few at a time, six to a dozen, up to roughly a hundred in a session. Breeding calls for very soft, acidic water and dim light, with peat filtration helping, so most keepers never spawn them, though it is doable with conditioned fish. The eggs hatch in about a day, and the fry take infusoria-grade food before moving on to baby brine shrimp.

Common problems

A hardy fish with few problems of its own. Ich is the usual complaint and tends to show up on newly bought fish stressed by transport. A washed-out, dull orange is a useful warning sign that something is off, whether high nitrate, an unstable temperature, or harassment from tankmates, since the colour fades under stress. The occasional fungal patch responds to standard treatment. Kept in stable, clean, soft water a harlequin can live five or six years, so fish dying at a year or two point to poor conditions rather than old age.

Bioload

Further reading

• Blackwater tanks: replicating acidic jungle streams
• The schooling number matters more than you think
• Stocking ideas by tank size