Threadfin rainbowfish

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.

Typically wild-caught; acclimate slowly.

Feeding

Small mouths. Micro pellets, crushed flake, frozen baby brine shrimp, frozen cyclops, frozen daphnia, and live food (baby brine shrimp, daphnia, microworms). Feeds at the surface and mid-water. Not competitive at feeding time; easily outcompeted by faster fish. Target-feed if necessary. Two to three small feedings daily.

Compatibility

• Delicate nano fish (4–5 cm) that's easily overwhelmed by active or aggressive tankmates. Best with other small, peaceful species in a planted tank.
• Males develop extraordinary fin extensions (the "threadfins") that are used in display toward females and rival males. These extensions are fragile and will be nipped off by any fin-nipper in the tank.
• Good companions: ember tetras, pygmy corys, celestial pearl danios, shrimp, and other calm nano species. Avoid barbs, danios, and anything fast enough to outcompete them or nippy enough to target the fins.
• Males display constantly when females are present. A group of 3 males and 5-6 females in a planted 60-liter tank is one of the most underrated small-tank displays in the hobby.

Origin and habitat

Iriatherina werneri, the threadfin rainbowfish, is a tiny, ornate rainbowfish, the only species in its genus, from the slow, plant-filled fresh waters of southern New Guinea and northern Australia. In New Guinea it ranges between the Merauke and Fly rivers and inland lakes, and in Australia across the Cape York swamps and the Northern Territory. Meinken described it in 1974 and named it after Arthur Werner, the German importer who helped collect the type fish from a rice paddy near Merauke. It belongs to the rainbowfish family, Melanotaeniidae. The body is grey with scales that flash blue, gold, and green, but the draw is the male's finnage: hugely elongated, thread-like rays on the dorsal and anal fins that he spreads and flicks in display, with an orange belly and reddish fins when in condition. Males reach about 4 cm and females a little under 3 cm. It is a peaceful, slightly delicate shoaler, and almost all trade fish are tank-bred; there is some suggestion the Australian and New Guinea populations are genetically distinct enough to be separate forms, though that is unresolved. IUCN lists it as Least Concern.

Breeding

An egg scatterer that lays among fine plants and moss, with no parental care. A male courts females with a ritualised dance of spread fins, and a female sheds a small number of eggs, on the order of half a dozen, each day over a long stretch, adding up to a couple of hundred in a month. The sticky eggs cling to plants or a spawning mop, and since the adults eat them, dense moss or moving the mop to a hatching container saves them. The eggs are slow to develop for an egg-scatterer, taking about a week to ten days, and the fry are minute, needing infusoria or liquid fry food before baby brine shrimp. In a planted species tank young appear on their own over time.

Common problems

The fine fin rays are the obvious vulnerability: they are easily nipped or torn and take weeks to regrow, so the fish must be kept only with gentle, non-nippy tankmates and handled carefully. It is sensitive to water quality, with little tolerance for ammonia or nitrite, and columnaris is the bacterial infection most likely to appear when conditions slip, while ich is a risk during acclimation. The warm-water preference, up around 26 to 30 C, narrows the choice of tankmates. A common letdown is appearance: the spectacular fins in photos take a mature male six months or more in good conditions and a mixed group to develop, so freshly bought juveniles look plain by comparison. With a short lifespan a colony needs to keep breeding to persist.

Bioload

Further reading

• How to cycle a fish tank
• Stocking ideas by tank size