Threadfin Rainbowfish (Iriatherina werneri)

Threadfin Rainbowfish — Nature’s Most Delicate Masterpiece

Gossamer-finned, iridescent, and breathtakingly delicate, the Threadfin Rainbowfish (Iriatherina werneri) is one of the most exquisite nano fish in the freshwater hobby. The males are particularly spectacular — their elongated, thread-like fin rays flutter like silk ribbons in the water, catching the light with every movement in a display of extraordinary natural beauty.

A small school of Threadfin Rainbowfish drifting through a lush planted aquarium is one of the most serene and visually stunning sights the hobby has to offer. These are fish that make you stop and stare.

Care at a Glance

• Common Name: Threadfin Rainbowfish, Featherfin Rainbowfish
• Scientific Name: Iriatherina werneri
• Origin: New Guinea & northern Australia (slow-moving, heavily vegetated waters)
• Adult Size: Up to 3 cm (1.2 inches) — a true nano fish
• pH: 6.0 – 7.5
• Temperature: 24 – 28°C (75 – 82°F)
• Temperament: Very peaceful; best in species-only or carefully chosen nano tanks
• Difficulty: Moderate

Tank & Setup Tips

Threadfin Rainbowfish are best suited to a calm, well-planted nano aquarium of 10 gallons or more. They are extremely sensitive to water flow — strong currents will stress them and damage their delicate fins. Use a sponge filter or a spray bar diffuser to keep flow gentle.

Dense planting with fine-leaved plants (java moss, hornwort, water sprite) and floating cover replicates their natural habitat and gives them security. A dark substrate and soft lighting will bring out their iridescent colours at their most vivid.

They are sensitive to poor water quality, so consistent water changes and stable parameters are essential. Avoid housing them with boisterous or fin-nipping species — their delicate fins are easily damaged.

Feeding

Micro predators in the wild. In the aquarium, they require very small foods — their tiny mouths cannot accept standard flakes or pellets. Feed baby brine shrimp, micro worms, daphnia, and high-quality micro pellets or crushed flakes. Feed small amounts 2–3 times daily.

Compatible Tank Mates

Best kept in a species-only tank or with very small, peaceful nano species that won’t compete aggressively for food or damage their fins. Suitable companions include small rasboras (chili, exclamation point), pygmy corydoras, small snails, and dwarf shrimp (with caution — adults are generally safe).

Breeding

Males display their spectacular fins in courtship dances to impress females. Eggs are scattered among fine-leaved plants and hatch in 7–10 days. Fry are tiny and require infusoria or commercial fry foods initially.

📸 Photos are for reference only. Actual fish may vary slightly in colour and pattern — each one is uniquely beautiful!