Threadfin rainbowfish
Iriatherina werneri
intermediate careOverview
The threadfin rainbowfish (Iriatherina werneri) is one of the most spectacular nano fish in the hobby. Males carry extravagant thread-like extensions on their fins that they fan out in constant, flag-waving displays. It’s a small, delicate species, and its beauty comes at the price of patience — this is a fish that rewards a careful keeper and punishes a rushed one. Do it right and a shoal of displaying males is unforgettable.
Tank & water
A group needs at least 60 litres (15 gallons), longer rather than taller, in a well-established tank.
- Mature, stable water: never add threadfins to a new setup. Cycle and mature the tank first — they are intolerant of ammonia and swings.
- Very gentle flow: their trailing fins hate current. Baffle the filter right down or use a sponge filter.
- Warmth: hold 23–28 °C with a reliable heater.
- Dense planting: fine-leaved and floating plants give security and encourage displays, and give any fry somewhere to hide.
- Water: soft to moderately hard and neutral, around pH 6.5–7.5, suits them well; avoid extremes and keep parameters rock-steady.
Feeding
A micropredator with a very small mouth, the threadfin needs tiny foods delivered patiently. Crushed micro-flakes, micro-pellets from the fish food range, and above all small live and frozen items — cyclops, baby brine shrimp, daphnia and grindal worms. Feed little and often so slow feeders get their share.
Tankmates
Only the calmest companions: pygmy corydoras, small rasboras, otocinclus, Pseudomugil blue-eyes and shrimp. Absolutely no fin-nippers, no fast greedy feeders, no bettas. Keep threadfins in a group of eight or more, ideally with several males, so they display to each other rather than sulk.
For a peaceful nano community built around them, browse gentle stocking ideas in our best bottom-dwelling fish answer.
Common mistakes to avoid
- Adding them to a new or unstable tank
- Strong filter flow damaging their fins
- Fast or nippy tankmates that outcompete or shred them
- Foods too large for their tiny mouths
Threadfin rainbowfish — frequently asked questions
Why do my threadfin rainbowfish keep their fins clamped?
Usually stress from boisterous tankmates, strong flow or unstable water. Males only spread their spectacular threadfin fins when they feel secure among their own kind and have rivals to display to. Give them a calm tank, gentle current and a large group.
Are threadfin rainbowfish good for beginners?
They are best for keepers with some experience. They are delicate, slow feeders that struggle to compete for food, need very small foods and mature, stable water. In the wrong tank they simply waste away.
Can threadfin rainbowfish live with bettas or fast fish?
No. They must never be housed with fin-nippers or fast, greedy feeders. Their long fins are targets and they are easily outcompeted. Keep only with equally peaceful, slow, small fish.
Found your model? Buy it at the right price.
UniverTrack tracks the real price of your aquarium gear across several retailers, spots fake discounts and warns you when it's genuinely the right moment to buy — with an AI assistant to guide you.