Threadfin Rainbowfish

About

The featherfin rainbowfish (Iriatherina werneri) is a tiny but visually elaborate fish from the slow-moving waterways, lily lagoons, and swampy billabongs of northern Australia and central-southern New Guinea. In the wild it stays close to dense vegetation, picking at diatoms and micro-crustaceans along heavily planted margins. Males carry the real visual interest here: the first dorsal fin is rounded and sail-like, while the second dorsal and anal fins trail long, thread-like filaments that the males flick and fan during courtship displays, intensifying in color as they compete for female attention. It is the only member of its genus, sitting somewhat apart from other rainbowfishes in both looks and structure. Despite being widely available in the trade, this is not a beginner's fish. It is genuinely delicate and easily outcompeted for food, and the long fins on males invite nipping from species that would otherwise be considered peaceful community fish. It does best in a dedicated setup alongside similarly calm, fine-mouthed companions like Pseudomugil species, pygmy corydoras, or small shrimp. Keep it in groups of at least six, ideally ten or more, where the fish settle, behave naturally, and males show their best colors. A well-planted aquarium with floating cover and stable water conditions gives this species the best chance to thrive.