Threadfin Rainbowfish Care Guide - Tank Setup, Diet & Breeding

Threadfin Rainbowfish are delicate, elegant fish named for the long, thread-like extensions on the dorsal, anal and pelvic fins of mature males. Their slim bodies often show a silvery, blue-green or golden sheen, while the fins can display striking red, orange, yellow and black markings. Under good lighting, their reflective scales and flowing finnage give them an unusually graceful appearance.

This species is peaceful and naturally sociable, spending much of its time in the middle and upper levels of the aquarium. Males frequently display to one another and to females by spreading their fins, producing an engaging courtship spectacle without normally causing serious injury.

In the wild, Threadfin Rainbowfish inhabit quiet, shallow waters among grasses, submerged vegetation and leaf litter. Their fine fins and relatively small mouths reflect a life spent picking tiny aquatic invertebrates and other small foods from plants and the water column.

Iriatherina werneri is a comparatively specialised rainbowfish and can be more sensitive than larger, hardier members of the family. Its understated body colour becomes far more impressive when the fish are settled, well conditioned and displaying in a suitable group.

Threadfin Rainbowfish come from the freshwater swamps, slow-moving streams and floodplain habitats of southern New Guinea and northern Australia, including parts of the Trans-Fly region and the Northern Territory. They are associated with shallow, plant-filled waters that may be stained by tannins and have limited current.

A group should have at least a 60-litre aquarium, with a footprint of roughly 60 by 30 cm or larger; extra length is valuable for swimming and displaying. Use gentle filtration and subdued flow, with dense plants around the edges, fine-leaved vegetation, floating cover and some open water in the centre. A dark, fine-grained substrate, subdued lighting and pieces of wood or leaf litter can help recreate their shaded, tannin-influenced habitat while making their colours stand out.

Males reach about 5 cm (2 inches) in total length, while females usually remain closer to 3.5–4 cm (1.4–1.6 inches).

Offer a varied diet of very small foods, including finely crushed quality flakes, micro pellets, baby brine shrimp, daphnia, cyclops and other tiny frozen or live foods. Their small mouths make appropriately sized food important, and live or frozen offerings are especially useful for bringing out their colour and conditioning them for spawning. Feed modest portions several times a day when possible rather than one large meal.

Threadfin Rainbowfish suit peaceful community aquariums with similarly calm species such as small tetras, pencilfish, peaceful rasboras, small Corydoras, Otocinclus and other gentle nano fish. Avoid fin-nippers, boisterous barbs, large fish and aggressive tankmates, all of which may harass these timid fish or damage the males’ extended fins. They can be kept with other peaceful rainbowfish if the aquarium is spacious and the species are not outcompeted for food.

Threadfin Rainbowfish are egg scatterers that can breed readily in a mature, planted aquarium. Provide fine-leaved plants, Java moss or a spawning mop for the pair to deposit their small adhesive eggs. Adults may eat the eggs, so move the spawning material to a separate container or remove the adults after spawning. The fry are tiny and should initially be fed infusoria or another suitably small liquid or powdered food before progressing to newly hatched brine shrimp.

Males are larger and much more ornate, developing elongated, filamentous extensions on the dorsal, anal and pelvic fins along with stronger colour and more pronounced fin displays. Females are smaller, shorter-finned and generally less intensely marked, with a rounder body when carrying eggs.