Threadfin Rainbowfish Breeding Guide

Breeding Iriatherina werneri: sexing, the male's flicking fin display, continuous egg-depositing into mops or moss, and rearing tiny fry.

Overview

Iriatherina werneri is a small, delicate rainbowfish, reaching about 5 cm in body length excluding the elongated tail fin, found in freshwater swamps and thickly vegetated flowing waters of tropical northern Australia and New Guinea. It is notable for the dramatically elongated fins of the male and an elaborate breeding display. Females deposit a few eggs daily over an extended period rather than spawning all at once, so a colony in a well-planted tank can produce fry continuously.

Sexing

Adult males possess extended unpaired and pelvic fins and are larger and more colourful than females.

Breeding Setup

Two approaches work. In the isolation method, place a single male with two or three females in a smaller tank fitted with an air-powered sponge filter and a spawning medium of nylon mops or aquatic moss. In the colony method, keep a group of adults in a larger, fully decorated, well-planted setup that allows some fry to survive on their own.

Spawning Behavior & Trigger

Displaying males intensify their colouration, hold the rounded first dorsal fin erect, and use a flicking gesture of the filamentous rays of the second dorsal and anal fins to entice a mate, often under floating vegetation. Spawning occurs throughout daylight hours in warmer periods, and a single male may mate with several females in one day, with females depositing a few eggs daily for several days.

Egg & Fry Care

Eggs scattered among submerged vegetation hatch within roughly 7-10 days. Rear the fry with regular small water changes and feed them at least twice daily. Their extremely small size at first feeding is the principal difficulty of artificial rearing.

Common Challenges

The very small first-feeding fry are demanding to raise, and the male's delicate filamentous fins mean tankmates that nip fins must be avoided in any breeding group.