Threadfin Rainbowfish Care Guide

The threadfin rainbowfish is a delicate, slow, tiny-mouthed nano fish whose spectacular thread-like male fins make it one of the most striking small fish in the hobby — and one of the easiest to lose. The real killer is not disease but feeding competition: it is a slow, methodical feeder with a mouth you can barely see, and faster tankmates will quietly starve it while it still looks fine. It is not a fish for a rough community, a boisterous tank, or a new, unstable aquarium.

Threadfin Rainbowfish at a glance

The sourced figures the welfare engine uses to judge Threadfin Rainbowfish — the parseable key facts.

Where it comes from

It comes from the slow, weedy, shallow waters of southern New Guinea and tropical northern Australia — slow-moving streams, freshwater swamps, lagoons and billabongs less than about 1.5 m deep, with clear, often tannin-tinged water, abundant vegetation and floating lily pads. That biotope is the care sheet. The still swamp-and-billabong origin is why it wants near-zero flow — it is a weak swimmer with long fins and does badly in current. The densely vegetated margins are why it wants a heavily planted tank with floating cover and open lanes for the males to display. And the warm, vegetation-choked shallows where it picks tiny prey from the water column are why it needs a mature, biologically stable tank and ultra-fine food.

Did you know?

• The "threadfin" is real and extreme: mature males trail filamentous dorsal and anal extensions that can exceed twice the body length, turning a 3–4 cm fish into a living banner of around 5–6 cm total length.
• It is the smallest rainbowfish in the hobby — the dwarf of the rainbowfish family, and the rare rainbow that fits a nano tank.
• It is the only fish in its genus: Iriatherina has been monotypic since Meinken described it in 1974.
• Its courtship is dance, not combat: males settle rivalry by posturing and flicking the fan dorsal under floating plants, and only fully colour up when there are rivals to perform for.
• It is a near-continuous, egg-scattering spawner that breeds year-round in good conditions yet eats its own eggs, with fry so tiny they need infusoria before they can take baby brine shrimp.

Tank size — and why

A 15 US gallon footprint (around 60 cm long) is the practical floor for a real group; a 20-gallon "long" is better. The driver is horizontal swimming and display room plus group size, not bioload — this is a tiny, low-waste fish. Long tanks beat tall tanks here: males need horizontal lanes to display and the school needs open mid-water swimming space. Keep the tank covered, because the species is reported to be prone to jumping.

As a guide, a 20-gallon tank comfortably suits about 8–11 Threadfin Rainbowfish as a single-species display, leaving room for tankmates.

See it to scale

Adult Threadfin Rainbowfish reach only about 4 cm (1.6 in) long — close to the size they are sold at, so what you see is roughly what you get. The catch is the group: a proper shoal still needs about a 15-gallon tank, around 51 cm long.

Adult size is sourced; tank length is approximate for a standard 15-gallon aquarium.

Water parameters in practice

Aim for the comfort band of about 24–28 °C, comfortable to roughly 30 °C; pH around 6.0–7.0, slightly acidic to neutral, within a tolerated 5.0–8.0; and soft-to-moderate hardness, around 5–12 dGH. The load-bearing point is stability rather than the precise reading: sudden shifts of even a couple of degrees or a quarter-point of pH stress this fish quickly, which is the practical reason it needs a mature, established tank and slow acclimation. It tolerates a fairly wide static envelope but crashes fast when water quality slips. One contested wrinkle worth knowing: although the adults are a soft-water species, several breeders report fry survival improves markedly in harder water, around 15–17 dGH — so keep the display tank soft-to-moderate but consider harder water in a dedicated fry tank.

Will it thrive in your water?

The comfortable range for Threadfin Rainbowfish is about 22–30 °C (72–86 °F) and pH 5.5–7.5. Test your own tap water against it below.

These are the sourced comfortable ranges. Stable water matters more than chasing an exact number — a steady reading inside the band beats a "perfect" one that drifts. Some fish also need a particular water hardness (GH); where that applies, the prose above covers it.

Diet & feeding

This is the species' number-one care trap. In the wild it is a micropredator and grazer of tiny items — diatoms, micro-crustaceans, insect larvae and unicellular algae — and it has a tiny, upturned, surface-directed mouth with a narrow throat, feeding in the upper water column. Standard tropical flake or community pellets are physically too large to swallow; the maximum practical food size is around 0.5 mm. Feed ultra-fine foods only: nano-grade micro pellets, powdered or finely-crushed quality flake, and small live or frozen items such as microworm, newly-hatched brine shrimp, sieved daphnia or moina, cyclops, copepods and vinegar eels. Just as important is how it eats: it is a slow, methodical feeder that picks prey out of the water column with deliberate strikes, so boisterous tankmates will out-compete it every time and the threadfins will quietly fade. Feed small portions two or three times a day, by day, using foods that stay suspended, and in a community feed in a calm spot or target-feed so they actually get their share. More threadfins die of quiet starvation in a competitive tank than of any disease.

Gear & setup

Flow is critical: keep it near-zero. An air-driven sponge filter is the gold standard, and any output should be baffled — males will not display in a tank where they have to swim against the current. Use a dark substrate to deepen colour, dense planting with floating plants for diffused light, and open swimming and display lanes between the planting (fine-leaved plants and moss double as spawning sites; the fish does not eat plants). A heater holds the warm setpoint, and the tank must be biologically mature, stable and cycled before these fish go in. Keep it firmly covered — it jumps.

Temperament & behaviour

A peaceful, timid, even shy shoaling species — easily scared but quick to recover — and crucially the victim in any conflict, never the aggressor: it does not nip at all. Its long male fins, on the other hand, are highly nip-vulnerable. Males compete not by fighting but by dancing and posturing — raising and lowering the fan-shaped dorsal and flicking in front of females and rival males — entirely without physical contact or injury. A good ratio of more females than males (say one male to two or three females, or multiple males with sightline breaks) keeps any one fish from being over-pursued. The colour and full finnage only switch on when there are rivals to display against; in too small a group, too busy a tank, or too much flow, the fish hide, wash out, stop displaying and slowly starve.

Group & social needs

A shoaling species that must be kept in a group. Six is the bare minimum; eight to ten or more is the real target. Larger groups reduce shyness, bring out colour, and — critically — give the males rivals to display against, which is what triggers their full finnage and colour over the first six to nine months. Skew the group towards more females than males so no individual is over-chased. Lone or paired fish are nervous and drab.

Compatible tank mates (preview)

A short, engine-cleared shortlist — the species TankStocking's welfare engine clears with Threadfin Rainbowfish and that suit its size and temperament best. Tap a name for its care guide, or use + to load the pairing in the planner.

• Amano Shrimp+Peaceful temperament, similar adult size, not a fin-nipper
• Assassin Snail+Uses the bottom zone, peaceful temperament, similar adult size, not a fin-nipper
• Black Neon Tetra+Peaceful temperament, similar adult size, not a fin-nipper
• Black Phantom Tetra+Peaceful temperament, similar adult size, not a fin-nipper
• Bolivian Ram+Uses the bottom zone, peaceful temperament, similar adult size, not a fin-nipper
• Bronze Corydoras+Uses the bottom zone, peaceful temperament, similar adult size, not a fin-nipper
• Cardinal Tetra+Peaceful temperament, similar adult size, not a fin-nipper
• Celestial Pearl Danio+Peaceful temperament, similar adult size, not a fin-nipper

A note on the shrimp and snails here: Threadfin Rainbowfish is peaceful and generally invertebrate-safe — but almost any fish will take very small shrimplets given the chance, so give shrimp dense cover (moss, leaf litter) if you want a colony to grow, rather than expecting every baby to survive.

This engine-cleared shortlist is Threadfin Rainbowfish's tankmate surface for now — a dedicated tank-mates guide can follow for high-demand species.

Breeding & sexing

Sexing is easy once the fish mature: males are larger and far more colourful, carrying the long fan dorsal, the thread-like dorsal and anal filaments and red-edged caudal lobes, while females are smaller, plainer and rounder-bodied with short, rounded fins and no filaments. Juveniles look alike until the males develop their finnage. Spawning itself is only moderately difficult — the real bottleneck is raising the microscopically small fry. It is a near-continuous egg-scatterer with no parental care that eats its own eggs and fry, spawning almost year-round in good conditions. Condition on live foods, warm the tank to around 28–30 °C, and provide fine-leaved plants, moss or spawning mops near the surface under floating cover; females deposit small numbers of adhesive eggs daily over many days. Eggs hatch in roughly five to ten days depending on temperature, and the fry are so tiny they need infusoria or rotifers for the first week or two before they can take vinegar eels and then newly-hatched brine shrimp.

Lifespan

A relatively short-lived rainbowfish: plan on about three to four years, with two years at the low end and roughly five at the well-kept high end. The species-specific killers are feeding competition and slow starvation, parameter and TDS instability (it crashes fast when water quality slips), nipped fins from the wrong tankmates, immature tanks, and the rapid progression of disease in such a small body.

Common mistakes

• Putting it in a busy or boisterous community. The number-one mistake: faster fish out-eat it and it slowly starves while still looking "fine." Calm tankmates only.
• Feeding standard flake or pellet. The mouth is too tiny — food must be powdered or crushed flake, sub-0.5 mm micro-food, or small live/frozen items.
• Housing it with fin-nippers. Tiger barbs, serpae or black-skirt tetras and rosy barbs will shred the male filaments, sometimes in a single afternoon.
• Too much flow. A weak swimmer that won't display against current — use a sponge filter and baffle any output.
• Too few fish, or the wrong ratio. Under about six they stay shy and drab, males won't colour up without rivals, and the group should skew to more females than males.
• Adding it to a new or unstable tank. It needs a mature tank and is sensitive to pH, hardness and TDS swings, so acclimate slowly.
• No lid. It jumps — keep it covered.

Signs of trouble

• A thinning body — the starvation tell, and the most common real-world cause of decline in a competitive tank.
• Clamped or eroding finnage and loss of colour — stress, often following nipping or unstable water.
• Hiding, not displaying and hanging away from the group — early social or environmental stress.
• Flicking, white spots or fungal/bacterial patches on damaged fins — disease moves fast in such a small body, so catching it in the first 24 hours can save the whole school.

Is this fish right for you?

Don't buy threadfins if you have a rough or fast-feeding community, any fin-nippers, a brand-new or uncycled tank, strong filter flow you can't tame, or if you can't supply ultra-fine food and keep a calm group of eight to ten. This is an intermediate-keeper or dedicated-nano fish, not a starter rainbow — Seriously Fish flatly calls it "not a recommended beginner's fish." On stock quality, the species is mostly tank-bred, so there is no dyed or balloon morph problem; the welfare red flags here are buying weak, non-feeding stock or already fin-damaged males, so choose healthy, actively-feeding, well-finned fish.

Bringing one home

Because it is sensitive to TDS and pH shock, acclimate slowly — a drip acclimation suits it well — and add it only to a biologically mature, stable, cycled tank, never a new one. Quarantine new arrivals for two to four weeks with sponge filtration, and during that time confirm they are actively feeding on ultra-fine food before moving them into the display tank.

Common questions

Are threadfin rainbowfish hard to keep?

Yes — they are delicate and not a beginner fish. The dominant killer is feeding competition: a slow, tiny-mouthed feeder, the threadfin is easily out-competed by faster tankmates and quietly starves. It also needs a mature, stable tank, near-zero flow, ultra-fine food, and calm tankmates that won't nip the males' long fins.

What do threadfin rainbowfish eat?

Ultra-fine foods only. The mouth is tiny and upturned, so the maximum practical food size is about 0.5 mm: nano micro pellets, powdered or finely-crushed flake, and small live or frozen items like microworm, newly-hatched brine shrimp, sieved daphnia and cyclops. Feed in a calm spot so they actually get their share.

What are good tankmates for threadfin rainbowfish?

Tiny, calm, slow-paced nano fish — small rasboras like chili and harlequin types, ember and other small peaceful tetras, Pseudomugil blue-eyes, pygmy corydoras and otocinclus, plus dwarf shrimp and snails. Avoid all fin-nippers, fast or boisterous feeders, and any larger or predatory fish.

Can threadfin rainbowfish live with fin-nippers like tiger barbs?

No. The males' long thread-like fins are highly nip-vulnerable, and fin-nippers such as tiger barbs, serpae tetras and rosy barbs will shred them quickly. The threadfin itself never nips — it is always the victim, so it needs peaceful tankmates only.

How many threadfin rainbowfish should I keep, and what ratio?

Six is the bare minimum; eight to ten or more is far better. Larger groups give the males rivals to display against, which is what triggers their full colour and finnage. Skew the group towards more females than males so no individual is over-pursued.

Sources & confidence

Sources & confidence (9 species)

These back the Threadfin Rainbowfish figures and the previewed tank mates above. Each figure is read from the TankStocking species database (v2026.06); below is the care reference behind it and how confident we are in that data. Confidence reflects the source quality, not whether any pairing is safe. Full source list and the welfare model are on the methodology page.

• Threadfin Rainbowfish Iriatherina werneri — Seriously Fish (seriouslyfish.com/species/iriatherina-werneri) high confidence
• Amano Shrimp Caridina multidentata — Aquarium Co-Op amano shrimp care; Aquadiction high confidence
• Assassin Snail Anentome helena (Clea helena) — The Shrimp Farm (theshrimpfarm.com/posts/assassin-snail-care) high confidence
• Black Neon Tetra Hyphessobrycon herbertaxelrodi — Seriously Fish / Aqua-Fish (Hyphessobrycon herbertaxelrodi) high confidence
• Black Phantom Tetra Hyphessobrycon megalopterus — Seriously Fish (Hyphessobrycon megalopterus) high confidence
• Bolivian Ram Mikrogeophagus altispinosus — Seriously Fish (seriouslyfish.com/species/mikrogeophagus-altispinosus) high confidence
• Bronze Corydoras Corydoras aeneus — Seriously Fish (seriouslyfish.com/species/corydoras-aeneus) high confidence
• Cardinal Tetra Paracheirodon axelrodi — Seriously Fish (seriouslyfish.com/species/paracheirodon-axelrodi) high confidence
• Celestial Pearl Danio Celestichthys margaritatus — Seriously Fish (seriouslyfish.com/species/celestichthys-margaritatus) high confidence

Care-guide sources (8)

This guide synthesises the references below; where they disagree, the range and the disagreement are noted in the text above. The figures in the key-facts box are read from the TankStocking species database (v2026.06). Full welfare model on the methodology page.

• Seriously Fish — Iriatherina werneri — authority (Meinken 1974), monotypic genus, family, type locality, range, biotope (<1.5 m vegetated margins), wild water (22–32 °C, pH 5.2–7.5), aquarium params (22–30 °C, pH 5.0–8.0, ~18–215 ppm, 60×30 cm), 30–40 mm SL, tiny-mouth diet, "easily outcompeted for food," "long fins… may be nipped," group of 6 ideally 10+, sexing, breeding, "quite delicate… not a recommended beginner's fish"
• FishBase — Iriatherina werneri — Meinken 1974, Melanotaeniidae, max 4.0 cm SL (male) / 3.0 cm SL (female), temp 26–30 °C (alt 23–28), pH 6.0–8.0 (alt 5.2–6.6), dH 5–12, distribution 5–10°S, IUCN Least Concern, trophic level 2.8, diet (diatoms/micro-crustaceans), 5+/60 cm husbandry note, longevity "2–3 years"
• Wikipedia — Threadfin rainbowfish — Meinken 1974, monotypic Iriatherina, range (N Australia + New Guinea), body to ~5 cm excl. tail / male total ~6 cm, male dimorphism, habitat, pH 6–7 / 23–29 °C, egg-scatter in vegetation, hatch 7–10 days, male courtship (raising/lowering dorsal), IUCN Least Concern
• Fishes of Australia — Iriatherina werneri — Australian distribution (Goyder R NT; Jardine R Cape York QLD), habitat (slow streams/billabongs, 23–28 °C), max 4 cm SL / commonly 3 cm, detailed fin/filament description, colour, dimorphism, year-round spawning, adhesive eggs, hatch 7–10 days, diet (insects/larvae, micro-crustaceans, algae)
• Shrimp and Snail Breeder (aquariumbreeder.com) — Threadfin Rainbowfish — size 3–4 cm (males to ~6 cm), lifespan 3–4 yr, temp 24–28 °C, pH 6–7, GH 7–19 (fry better in 15–17), 15 gal / 6–8, surface-feeder/upturned mouth, "very slow"/"pushovers" out-competed, fins >2× body, dancing/no contact, "do not nip," tankmate lists, jumping, breeding (~7 eggs/day, hatch 5–6 days), planted-safe
• aqua-fish.net — Threadfin Rainbowfish Care — size 4–5 cm, lifespan 3–5 yr, temp 24–29 °C, pH 6.0–7.5, hardness 2–12 dH, 60 cm min for a group, group 8–10, gentle flow, tiny-mouth ultra-fine feeding, sexing, continuous/mop breeder, avoid fin-nippers/boisterous/large fast feeders
• fishstores.org — Threadfin Rainbowfish Care Guide — size 4–5 cm, lifespan 3–5 yr, temp 23–28 °C, pH 6.0–7.5, 5–12 dGH, stability ("2–3 F or quarter pH point" stresses them), 15 gal / 20-long, near-zero flow / sponge filter, "males won't display against output," 8–10 school, fin-nipper warning, feeding-competition as "most common cause of slow decline," <0.5 mm food, breeding, "not a beginner fish," disease-moves-fast, quarantine 2–4 wks
• Aquatic Arts — Threadfin/Featherfin Rainbowfish — size 3–4 cm, peaceful / school of 6+, temp 22–30 °C, pH 5.0–8.0, KH 7–10, dwarf-shrimp safe, "one of a few exceptions" small enough for nano setups, jumping risk / cover, slower water / less turbulence, tank-bred sourcing

More on Threadfin Rainbowfish

Related guides on TankStocking — each scored by the same welfare engine as the planner.

This care guide is a sourced planning reference, not veterinary advice — individual fish, filtration and maintenance all matter. Cycle the tank, test your water, and observe your fish. How TankStocking works →