Threadfin rainbowfish

Iriatherina werneri

FreshwaterFish

Care at a glance

Temperature: 72–86°F (22–30°C)
pH: 6.0–7.5
Adult size: 1.6 in (4 cm)
Minimum tank: 13 gal (50 L)
Temperament: Peaceful
Social behavior: Schooling
Swim level: Middle
Difficulty: Medium
Breeding: No record
Rarity: Rare
Origin: New Guinea, Australia
Family: Melanotaeniidae

Overview

Threadfin rainbowfish are delicate, peaceful fish best kept in a group of eight or more, ideally with several males so they display their long, trailing fins to one another. They swim in the middle of the tank and need calm, slow tankmates, since faster fish will outcompete them at feeding time and may nip their flowing finnage. They have small mouths, so offer fine foods such as crushed flake, micro pellet, frozen cyclops, daphnia, and baby brine shrimp. Plenty of fine-leaved plants gives the males space to show off. Aim for a tank around 50 liters or larger.

In the wild

This species lives in northern Australia and southern New Guinea, in clear, still, and slow-moving waters such as swamps, lagoons, and vegetated stream edges. Those waters are warm and run soft and slightly acidic to neutral, matching a pH of about 6 to 7.5 and a temperature between 22 and 30 degrees Celsius. Recreate the habitat with fine substrate, dense fine-leaved and floating plants, some leaf litter, and calm, clean water with very gentle flow.

Good to know

Iriatherina werneri is a rainbowfish in the family Melanotaeniidae and the only species in its genus. Its body reaches only about 4 centimeters, but mature males extend long, thread-like dorsal and tail fins that make them look much larger. It scatters small eggs among fine plants and mops over an extended period, giving no parental care, and the fry hatch very small and need tiny live foods in their first days.

Did you know

The male threadfin rainbowfish carries some of the longest, most elaborate fins in the whole rainbowfish family, spread wide in display like small flags. Those trailing threads are also fragile, which is why calm tankmates matter.