Iriatherina werneri
The peacock of nano rainbowfish. Males display extended threadlike dorsal and anal rays, fanned out in territorial display every few minutes. Wants warm tropical water and absolutely no boisterous tank mates, they retract and refuse to display.
Threadfin Rainbowfish (Iriatherina werneri) reaches 4–5 cm as an adult and needs a minimum tank of 60 L. Native to Northern Australia, southern New Guinea, it lives in the mid to top water column with a peaceful temperament. Aim for 24–30 °C, pH 6.5–7.5, and 5–15 dGH hardness. Lifespan is 3–4 years with good care. Keep threadfin rainbowfish in groups of 6+, yes schoolers need numbers to display natural behaviour. Diet: micropredator, Micro pellets, daphnia, baby brine shrimp, microworms. Tiny mouth, keep food fine. Plant-safe: Yes. Shrimp-safe: Yes (adult shrimp).
Care at a glance
The peacock of nano rainbowfish. Males display extended threadlike dorsal and anal rays, fanned out in territorial display every few minutes. Wants warm tropical water and absolutely no boisterous tank mates, they retract and refuse to display.
By Mike ElmiraUpdated 2 min read
Part of our complete guide to aquarium fish for the planted tank.
The parameters that decide whether threadfin rainbowfish fits in your tank.
Mid to Top
Yes
Group of 6+
Melanotaeniidae
Micropredator
Micro pellets, daphnia, baby brine shrimp, microworms. Tiny mouth, keep food fine.
3–4 yrs
Medium
Calm tannin-stained creeks of Cape York, Australia
Northern Australia, southern New Guinea
Tank-mate safety and the species this one is documented to thrive (or fail) alongside.
Calm peaceful nano fish: ember tetras, chili rasboras, sparkling gouramis, otocinclus, cherry shrimp adults. Pseudomugil species (other blue-eyes).
Aggressive species, fin-nippers (any), boisterous mid-water fish that startle males into hiding. Larger gouramis that intimidate them.
Hard-won lessons from the tank.
Two males in a 60 L tank with 4 females produces near-constant display sparring — males flare fins at each other every 30–60 seconds. The 'flag waving' is purely showy; no actual aggression. A single male alone displays less because there's no audience.
Genus 'Iriatherina' = 'Iris of Athena' (the rainbow-eyed goddess). Species 'werneri' honours German fish collector Friedrich Werner.
What can go wrong and how to spot it.
Failure modes, in order of how dramatic the fix is.
Sensitive to nitrate above 20 ppm and rapid temperature swings. Susceptible to fin damage from boisterous tank mates.
Often kept too cool (22–24 °C) where they survive but lose display behaviour. They're tropical species — 27 °C is the sweet spot for male display intensity.
The practical routine, read top to bottom.
60 L+ with calm low-flow setup. Heavy planting with open mid-water swimming space for male display. Surface plants for shade. Warm temperature 26–28 °C, soft to moderately hard water. Sponge or low-flow canister filter.
3 weeks at appropriate temperature. Provide cover.
Where it comes from, how it behaves, and the variants you'll see at retail.
Where it lived before it came home.
Slow tropical pools and swamps across northern Australia, southern New Guinea, and the Aru Islands. Soft to moderately hard, warm, vegetated water with abundant cover.
Insect larvae, small crustaceans, biofilm. Active surface feeders.
Not threatened. Wild-caught and captive-bred both common in trade.
How they pair, reproduce, and grow.
Strong dimorphism. Males develop extended thread-like rays on dorsal and anal fins, plus a yellow-and-black fan-shaped tail. Females are smaller with rounded fins and pale silver-yellow colour.
Continuous egg scatterers. Soft to moderately hard water at 26–28 °C, dense moss or spawning mop. Eggs hatch in 7–10 days; fry are tiny and need infusoria for the first 10 days. Many fry survive in a heavily-planted display tank.
The named cultivars and the lookalikes worth flagging.
Wild type. No common cultivars; some breeders select for extra-long finnage males.
Direct answers to the questions search engines and AI assistants surface most often about this species.
Threadfin Rainbowfish (Iriatherina werneri) needs a minimum tank of 60 L. They live in the mid to top water column and should be kept in groups of 6+, so a longer footprint matters more than depth.
Target 24–30 °C, pH 6.5–7.5, and 5–15 dGH hardness. Acclimate slowly when moving them between water sources.
Shrimp safety: Yes (adult shrimp). Plant safety: Yes.
Threadfin Rainbowfish are micropredator. Micro pellets, daphnia, baby brine shrimp, microworms. Tiny mouth, keep food fine.
On Fin & Stem's 1–5 difficulty scale this species rates 3/5. Intermediate, stable parameters and a mature tank matter. Breeding difficulty: medium.
Typical lifespan in a well-maintained tank is 3–4 years.
A planted tank is a system. Pair this fish with one entry from each other pillar to plan the whole scape.
