Threadfin rainbowfish
Small freshwater fish with long, elegant fins from Australia and New Guinea.
The threadfin rainbowfish is a small, peaceful species native to tropical northern Australia and New Guinea. It is prized in the aquarium trade for its striking elongated fins and vibrant male colouration. First described in 1974, it thrives in vegetated freshwater habitats and requires specific care in captivity.
- Only species in the genus Iriatherina
- Males have larger and more colourful fins
- Eggs hatch after 7 to 10 days
- Prefers groups of six or more fish
- Requires live foods due to small mouth size
What it is
The threadfin rainbowfish is the sole member of its genus and stands out among rainbowfishes for its exceptionally long and delicate fins. Adults reach about 5 cm in body length, though their trailing fin rays extend much further. Males display more intense colours and larger finnage than females, making sex identification straightforward. This species is considered one of the most visually appealing members of its group.
Where it lives
This fish inhabits tropical regions of northern Australia and New Guinea. It is found in freshwater environments, specifically preferring swamps and areas with dense vegetation in flowing waters. The type specimens were originally collected in Merauke, Irian Jaya, Indonesia. Populations exist in northern Queensland and the Northern Territory, as well as across New Guinea.
How it lives
Threadfins are peaceful creatures that thrive in groups of six or more individuals. Due to their tiny mouths, feeding can be challenging, and they prefer live foods like baby brine shrimp or microscopic larval diets. In the wild, they occupy demersal zones thick with plant life. Genetic studies suggest potential divergence between populations in different Australian territories and New Guinea.
Breeding and life cycle
Spawning involves scattering eggs among submerged vegetation leaves. These eggs typically hatch within seven to ten days. In captivity, males court females by rapidly raising and lowering their forward dorsal fin. Dominant males exhibit polygamous behaviour, so a ratio of one male to three females is recommended for breeding tanks. Best results occur in species-only setups.
Fish and people
Aquarists value this species for its beauty but note its specific care requirements. They should be kept in tanks of at least 20 gallons, with 30 gallons preferred, and housed away from fin-nipping tank mates. Water conditions should be soft to moderately hard, slightly acidic, and warm. The species name honours Arthur Werner, the trader who collected the first specimens.
Questions people ask about Threadfin rainbowfish
How large do threadfin rainbowfish grow?
They grow up to 5 cm in body length. This measurement does not include their long, trailing tail filaments which extend significantly beyond the body.
What water conditions do they need in an aquarium?
They prefer soft to moderately hard water that is slightly acidic, with a pH between 6.0 and 7.0. The temperature should be maintained between 23 and 29 degrees Celsius.
Can they live with other fish species?
They are peaceful but should not be kept with fin-nippers due to their long fins. They breed best in species-only tanks, though they can live in community tanks if tank mates are gentle.
Good reading and sources
- Threadfin rainbowfish on WikipediaEncyclopedia
- More photos on Wikimedia CommonsPhotos
- Wikidata recordOpen data
The facts on this page come from the Wikipedia article "Threadfin rainbowfish", written by its contributors and used under the CC BY-SA 4.0 licence, and from its Wikidata record (CC0). EveryFish rewrote the text in its own words, with AI assistance, and compares every page with the article to make sure no passage is copied. It may be reused under the same CC BY-SA 4.0 licence with a link back.
It may contain mistakes, and figures change over time. Nothing here is investment or buying advice. If something is wrong, tell us. Last refreshed 2026-11-15.
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