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

Native to central-southern New Guinea and northern Australia. In New Guinea, distributed along the southern coast between Merauke and the Fly River (recorded over 500 km inland), including Lakes Bosset and Kala; in Australia, found in river basins of the Cape York Peninsula (Queensland) and the Northern Territory including the Arafura wetlands of Arnhem Land. Described by Meinken in 1974, named after Arthur Werner, a German aquarium fish importer who helped collect the type specimens from a rice paddy near Merauke. The sole member of its genus; Iriatherina has remained monotypic since description. Placed within the family Melanotaeniidae. Prefers slow-moving streams, freshwater swamps, lagoons, and densely vegetated still waters with clear water. Recorded at 26-30°C and pH 6.0-8.0 in the wild. Males reach about 4 cm standard length; females about 3 cm. Males develop dramatically elongated rays on the dorsal and anal fins used in display; the body is gray with reflective scales gleaming blue, gold, and green, and males in breeding condition show orange belly and reddish fins. Wild examples are virtually unknown in the hobby; all trade specimens are commercially bred. Populations from Australia and New Guinea may exhibit enough genetic separation to warrant recognition as distinct taxa, though this is unconfirmed. Lifespan roughly 3-5 years. IUCN Least Concern.

Breeding

Egg scatterer that deposits eggs among fine-leaved plants and moss. Males display to females with spread fins, hovering and darting in a ritualized dance. The female deposits a few eggs daily among moss or spawning mops. Eggs are small and adhesive. Adults may eat eggs they find, so dense plant cover is important. Eggs hatch in 7-10 days (slower than most egg-scatterers). Fry are tiny and need infusoria or liquid fry food for the first week, then baby brine shrimp. Growth is slow. In a species-only planted tank with thick moss, fry appear spontaneously. Dedicated breeding uses removable spawning mops that are transferred to a hatching tank every few days.

Common problems

Fin damage from nipping tankmates or rough handling. The filamentous fin rays are the fish's main visual feature and take weeks to regrow when broken. Keep with non-nippy species only. Sensitivity to water quality: the species doesn't tolerate ammonia or nitrite spikes well. Ich is a risk during acclimation. Columnaris (bacterial) is the most common infection in suboptimal water quality. The warm-water requirement (26–30°C) limits compatible species choices. Short lifespan (3-4 years) means the colony needs to breed to sustain itself. The biggest non-medical issue is that people buy them for the dramatic photos and are disappointed when the fins aren't as developed as the pictures. Full fin extension in males takes 6+ months of maturity and good conditions.

Bioload

Further reading

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