Threadfin Rainbowfish
Iriatherina werneri
Also known as: Threadfin rainbow, Featherfin rainbowfish
Quick Facts
- Adult size: 4-5 cm (1.6-2 inches)
- Minimum tank size: 60 litres (16 gallons)
- Lifespan: 2–4 years
- Temperament: peaceful
- Swimming level: middle-and-top
- Diet: omnivore
- Minimum group size: 6
- Difficulty: intermediate
Water Parameters
- Temperature: 23–30°C
- pH: 6–8
- Hardness (GH): 36–214 ppm (2–12 dGH)
Values above are general hardness (GH). For carbonate hardness (KH), the buffer that keeps pH stable, see the water parameters guide.
Care Summary
Among the most delicately beautiful of all small aquarium fish: a 4cm rainbowfish native to northern Australia and southern New Guinea, the males trailing extravagant black thread-like dorsal and anal fin extensions that they flick and fan in constant display against a body of soft red and blue. That beauty comes with specific needs. The threadfin has a minute mouth and is a slow, deliberate feeder, so it requires small foods and tankmates calm enough not to beat it to every meal, and its long fins make it a magnet for nippers. Short-lived even when well kept, it rewards a mature, gently-filtered, densely-planted tank and a group of its own kind. A genuine connoisseur's nano fish, and a lovely showcase of Australia's native rainbowfish.
Tankmates
Exquisite but delicate in compatibility terms: the threadfin rainbow is a slow, peaceful, tiny-mouthed fish with long trailing fins, which makes it vulnerable on two counts. Fast or greedy tankmates (even guppies and danios) outcompete it at feeding so it slowly starves, and any fin-nipper shreds the very finnage that makes it worth keeping. Keep it only with small, calm, slow-to-moderate feeders: pseudomugil blue-eyes, celestial pearl danios, small rasboras, pygmy corydoras, and dwarf shrimp, with which it is entirely safe. Males need a group, and ideally a few rival males, to display their full extended finnage. Provide gentle flow, fine foods, and a planted, mature tank.
Compatible with
- Celestial Pearl Danio
- Ember Tetra
- Chili Rasbora
- Pacific Blue Eye
- Pygmy Corydoras
- Cherry Shrimp
- Otocinclus
Avoid keeping with
- Betta
- Guppy
- Tiger Barb
- Serpae Tetra
- Fin Nippers
- Any Larger Fish
Common Problems
- starvation when outcompeted by faster feeders
- nipped fin extensions from inappropriate tankmates
- short lifespan (typically 2 to 3 years)
- preference for small live or frozen foods
Sources: seriouslyfish.com, fishbase.org · Last updated: 2026-06-20