Threadfin Rainbowfish (Iriatherina werneri)

The Threadfin Rainbowfish (Iriatherina werneri) is one of the most spectacular small fish in the freshwater hobby. Males carry absurdly long filamentous rays trailing from the second dorsal and anal fins, tipped in red and black, plus a lyre-shaped tail with red-edged lobes. When two males square off they flare everything at once and flick those threads at each other, and a group of them in a planted tank does this all day long. The body is barely an inch and a half, so all of that display is packed into a genuinely tiny fish. These are grown-out, medium-size fish, and all commercially available stock is captive bred. Browse the rest of our live fish and freshwater supplies to build the right tank for them.

Care Guide

• Tank size: a 20 gallon or a 24 by 12 inch footprint is the sensible minimum. They are small but they are constantly moving, and males need room to spread out and display.
• Group size: six is the floor, ten or more is where they come alive. Include several males. A group of males displaying to each other is the entire reason to keep this species, and a lone male never shows his best.
• Food must be tiny: crushed flake, micro pellets, nano pellets, baby brine shrimp, and daphnia. Live and frozen small foods bring out the best colour. Feed small amounts more often rather than one big feeding.
• Water parameters: 72 to 86°F, pH 5.0 to 8.0, hardness anywhere from very soft to moderately hard. Genuinely adaptable, which is why they suit so many setups.
• Flow: gentle. A sponge filter is ideal. Strong current pins those long fins flat and stops the display.
• Aquascape: dense planting around the edges with clear open water in the middle for displaying. Java moss and floating plants such as frogbit give them the dappled light they like.
• Lid: recommended. They are small surface-oriented fish and can jump when startled.
• Breeding: females scatter a few eggs daily among fine-leaved plants or spawning mops over several days. Adults largely ignore the eggs, but the fry are very small and delicate and need powdered food or live infusoria plus frequent small water changes. Raising them is the hard part, not getting them to spawn.

Specifications

Compatibility

Peaceful with everything, but easily bullied off its food, so the tankmate list is shorter than you would expect for such a harmless fish. The best companions are other small, slow, gentle species: Paskai Rainbowfish and other blue eyes are the classic pairing, along with Pygmy Cories, Celestial Pearl Danios, Ember Tetras, and dwarf shrimp.

Two things to avoid. First, anything fast and greedy: barbs, most danios, and larger tetras will clear the food before a threadfin has decided to eat. Second, fin nippers. Those trailing filaments are irresistible to a nipper, and once they are bitten off the fish loses the feature you paid for. Larger rainbowfish such as Boesemani are relatives but far too big and boisterous to share a tank with these.

Recommended Foods & Supplies

• Hikari Micro Pellets, sized for a mouth this small
• Xtreme Nano Slow-Sinking Pellets for slow deliberate eaters
• SFBB Baby Brine Shrimp and Freeze-Dried Daphnia for colour and conditioning
• Hikari Bacto-Surge Sponge Filter for gentle, low-flow filtration
• Java Moss and Frogbit for cover, spawning sites, and softened light

FAQ

How do I tell males from females?
It is obvious once they mature. Males have the long trailing filaments on the dorsal and anal fins, extended pelvic fins, more colour, and slightly larger size. Females are plainer silver with short, ordinary fins. The fish in the photo is a male.

Should I buy mostly males then?
Get a mix, but make sure there are several males. Males display hardest when they are competing with each other and courting females, so a group with both sexes gives you far more action than males alone.

Are they hard to keep?
Not once you understand the one issue. Their water requirements are wide and forgiving. The problem is purely that they have tiny mouths and feed slowly, so they need small food and calm tankmates. Solve that and they are undemanding.

Why do the long fins look short on the fish in the store tank?
Young males have not grown them out yet, and filaments can also be worn down in a crowded holding tank. They regrow in good conditions with proper feeding, so a male with short threads today will usually develop them fully in your tank.