Threadfin Rainbow

Threadfin Rainbow (Iriatherina werneri) is one of the most ethereally beautiful fish in the freshwater hobby. Males develop extraordinarily long, gossamer-thin fin extensions on the dorsal, anal, and caudal fins — thread-like filaments that trail as the fish moves, creating a delicate, flowing silhouette unlike almost any other freshwater fish. The overall effect is a fish that looks as though it were made of glass and silk.

Feeding & Care Tip: Sera Vipan Baby is the ideal daily staple — crush well before feeding as this species has a very small mouth. Supplement 2–3× per week with Hikari Frozen or Freeze-Dried Baby Brine Shrimp, which are an ideal size. Feed small amounts multiple times daily. This species has a small stomach and benefits from frequent small feedings over one large meal.

Native to New Guinea and northern Australia, inhabiting slow-moving, calm, heavily vegetated streams and billabongs. Iriatherina werneri is the only species in its genus. The delicate threadfin extensions are extremely fragile — they can be shredded by even gentle fin-nippers or strong water currents. This fish requires very specific conditions to thrive.

Keep only with other very small, very peaceful species that will not nip at the delicate fin extensions. Even fish that are not habitual fin-nippers may investigate the trailing threads. Suitable tankmates include other Pseudomugil, small rasboras, and micro corydoras. Avoid any current stronger than the gentlest sponge filter — strong flow shreds the threadfin extensions over time.

A densely planted aquarium with absolutely still to very gentle water movement, dark substrate, and no fin-nippers is essential for this fish to display its full beauty. Keep in groups of at least eight with multiple males to encourage constant display. Males fan their extraordinary fins in competition — the combined effect of several males displaying simultaneously in a calm, planted tank is one of the most remarkable sights in freshwater fishkeeping.

Care & Ideal Parameters