🐟 AquaticCare difficulty: IntermediateLegal complexity: Low
Threadfin rainbowfish
Iriatherina werneri · also called Featherfin rainbowfish, Werner's rainbowfish, Threadfin rainbow
A delicate nano rainbowfish whose males display spectacular extended, thread-like fins they flick and fan in courtship; gorgeous and peaceful but slow, small-mouthed, and easily outcompeted, so it needs careful tankmate choice.
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Quick facts
| Size | Tiny, with a body of about 4-5 cm (1.5-2 in), though males' extended fin filaments make them appear larger. |
| Lifespan | 3–4 years |
| Social needs | group |
| Native region | Northern Australia and southern New Guinea (slow, vegetated freshwater habitats) |
| Origin | Old World |
| Climate | 🌴 Tropical |
| Water type | 💧 Freshwater |
| Family | Melanotaeniidae |
| Genus | Iriatherina |
Part of the Rainbowfish
Iridescent, peaceful shoaling fish from Australasia and Sulawesi prized for their shifting colors; they shine in mature, well-planted aquariums kept in groups.
Habitat & space requirements
From the minimum an animal needs to be kept humanely, up to the ideal setup. Bigger is almost always better — minimums are floors, not targets.
Life & growth stages
How this animal changes through its life — each stage often has its own care, diet and space needs.
Habitat & enclosure
Threadfin rainbowfish are small, slow-moving, and easily intimidated, so they do best in a calm, well-planted tank of at least 60-75 L (15-20 gallons) for a group, with gentle flow they can swim against comfortably. They prefer warm, soft to moderately hard, slightly acidic to neutral water: temperature 24-30 C (75-86 F), pH 5.5-7.5, and soft to medium hardness. Strong current and turbulent water stress them and tatter the males' delicate fins.
Dense planting, including fine-leaved plants and floating cover, gives them security and the open mid-water lanes where males display. A darker substrate, calm conditions, and good but not harsh lighting help them feel safe and show their finnage. They are small jumpers, so keep the tank covered, and provide very stable, mature water as they dislike sudden changes.
Substrate
A dark fine sand or gravel substrate enhances their subtle colors and the males' fins. Plant densely with fine-leaved species and add floating plants for security, leaving calm open mid-water lanes where males display.
Equipment & setup
Use gentle filtration that keeps water clean without strong current, such as a sponge filter or a baffled outflow, plus a heater for the warm temperatures they like. Soft, planted lighting and a secure lid complete the setup; gentle flow is essential to protect the males' delicate finnage.
Diet
Threadfin rainbowfish have tiny upturned mouths and are specialist micro-feeders, which is the main challenge of keeping them: food must be small enough for them to take. Offer finely crushed flake, micro-pellets, and especially small live and frozen foods such as baby brine shrimp, microworms, daphnia, cyclops, and grindal worms. Vinegar eels and other tiny live foods are excellent for conditioning.
Feed several small meals a day and make sure faster tankmates are not stealing everything before the slow threadfins reach it. Live and frozen micro-foods bring out their best color and finnage and are important for breeding condition. Underfeeding, or feeding particles too large, is a frequent cause of decline in this species.
Behavior & temperament
Threadfin rainbowfish are exceptionally peaceful, gentle, and shoaling, and should be kept in groups of six or more with several males so they spread their finnage in harmless competitive displays. They are far too slow and timid for boisterous tankmates and should be paired only with equally calm, small, non-nipping fish such as small rasboras, pygmy corydoras, ember tetras, and peaceful dwarf shrimp; fin-nippers like tiger barbs will quickly ruin the males' fins. Males flick and fan their threads at one another and at females in a constant, beautiful display.
In a peaceful, planted tank with enough males they are active and confident; in the wrong company they hide and waste away. Enrichment comes from a calm planted environment with open display space and gentle flow. Their delicate beauty rewards a carefully chosen, low-aggression community.
Health
Threadfins are delicate and short-lived, and the most common problems are slow starvation from being outcompeted or fed too-large foods, fin damage from boisterous or nipping tankmates, and stress from strong flow or unstable water. They are also susceptible to ich and bacterial or fungal infections, and torn fins can become infected if water quality is poor. Their small size makes them sensitive to water-quality swings.
Prevent problems with very stable mature water, gentle flow, appropriately tiny foods offered often, peaceful tankmates, and quarantine of new fish. Watch for thinning bodies, frayed or fungused fins, white spots, clamped fins, and lethargy, and consult an aquatic veterinarian before medicating these sensitive fish. This information is general guidance and not a substitute for advice from a qualified aquatic veterinarian.
Tips, DIY & hacks
Match foods to their tiny mouths, leaning on baby brine shrimp and other micro-live foods, and keep only the most peaceful slow tankmates so the males' threads stay intact. Keep several males to trigger constant displays; for breeding, condition on live foods and use fine-leaved plants or spawning mops in soft, warm, gentle water.