Iriatherina Threadfin Rainbowfish

Iriatherina werneri is unlike any other rainbowfish in the hobby. Adult males develop astonishingly elongated thread-like extensions on the dorsal, anal, and ventral fins — silky, flowing filaments that extend several centimetres beyond the body and wave in the water column like gossamer ribbons during courtship. Combined with the iridescent silver body and contrasting black and red fin markings, the result is one of the most visually spectacular nano fish in freshwater aquaristics.

Found in shallow, warm, heavily vegetated waters of Papua New Guinea and northern Australia, Iriatherina werneri requires a gentle, slow-moving aquarium environment. Strong currents damage the delicate fin filaments and stress the fish. A thickly planted aquarium with floating plants to diffuse light and gentle sponge or box filtration provides ideal conditions. It is sensitive to poor water quality and requires frequent small water changes.

Despite its delicate appearance, I. werneri is not particularly difficult to keep if its requirements are respected. It breeds readily on fine-leaved plants and spawning mops, and its fry — while very small — can be raised on infusoria and commercial fry foods. The thread fins of males begin to develop at around three to four months of age.

Water: pH 6.0–7.5, temperature 24–28°C, soft to moderately hard; gentle filtration only; frequent small water changes. Tank: Minimum 40 L; heavily planted with floating cover; absolutely no strong currents that damage fin threads. Feeding: Micro-pellets, baby brine shrimp, micro-worms, and daphnia nauplii; feed small amounts multiple times daily. Breeding: Spawns on fine plants or mops; fry very small; infusoria or commercial fry food for first 10 days. Compatibility: Only with very small, gentle nano species; completely incompatible with fin-nippers of any size.