Threadfin Rainbowfish Care Guide: Tank Size, Diet & Tank Mates

Species Overview#

Threadfin rainbowfish (Iriatherina werneri) are the most ornate of the true nano rainbows. The males carry trailing dorsal and anal fin filaments that look painted on, ending in delicate threads that drift behind them as they swim. They come from slow, plant-choked backwaters of New Guinea and northern Australia, and they bring that environment with them — a healthy threadfin tank is heavily planted, gently filtered, and quiet enough that the fish never have to fight the current.

This is not a beginner fish despite its small size and modest tank requirements. The mouth is tiny, the fins are delicate, and the species is fast to crash if water quality slips. Aquarists who already keep nano communities and understand small-particle feeding will have the best results.

Adult size

1.5-2 in (4-5 cm)

Lifespan

3-5 years

Min tank

15 gallons

Temperament

Peaceful

Difficulty

Intermediate

Diet

Micropredator / omnivore

Natural Habitat#

Threadfins live in slow-moving, heavily vegetated waters across southern New Guinea and the Cape York Peninsula of northern Australia. The habitat is shallow, tannin-stained, and packed with submerged and floating plants — pandanus root tangles, water lilies, and dense beds of fine-leaved vegetation. Flow is near zero. Water is soft to moderately hard, slightly acidic to neutral, and warm year-round.

The takeaway for aquarists: replicate that calm, vegetated environment and the fish thrive. Try to run them in a high-flow community tank and they spend their energy fighting the current instead of displaying.

Appearance and Sexual Dimorphism#

Males are unmistakable. The first dorsal is a small triangular sail, the second dorsal and anal fins extend back into long, dark, thread-like filaments tipped with red, yellow, and black. The body shows iridescent blue-silver scales with bronze and red highlights along the flanks. Females are plainer — silvery body, short rounded fins, no filaments — but still attractive in a school context.

Juveniles of both sexes look similar. Males develop their filaments and color over the first 6-9 months in good conditions, and only when kept in groups with rivals to display against.

Size and Lifespan#

Adults reach 1.2 to 2 inches (3 to 5 cm), with males slightly larger thanks to their extended fins. Body length itself rarely exceeds 1.5 inches. Lifespan in a clean, properly fed tank runs 3 to 5 years. They reach sexual maturity inside of a year.

Water Parameters & Tank Requirements#

Threadfins are not difficult to please on numbers, but they are unforgiving of instability and any sustained nitrate above 20 ppm. The single biggest mistake is putting them into a tank that looks ready but has not fully cycled.

Ideal Water Parameters#

The species tolerates a soft, acidic-to-neutral range that lines up with its tannin-stained native waters. Slightly harder, neutral water also works as long as the change is gradual. Stability matters more than the precise reading — sudden shifts of even 2-3 F or a quarter point in pH stress them quickly.

Minimum Tank Size and Flow#

A 15-gallon tank is the realistic minimum for a school of six or more threadfins, with a 20-gallon long preferred for the extra horizontal swimming space and easier male territorial display. The footprint matters more than the gallon number — long and shallow beats tall and narrow for this species.

Critical: flow must be near zero at the surface. Threadfins do not handle current well, and males will not display in a tank where they have to swim against output. An air-driven sponge filter is the gold standard. Hang-on-back filters work only if the output is heavily baffled with a sponge or filter floss to break the surface ripple.

Planting and Décor#

Aquascape with dense planting in three layers. Floating plants on top — frogbit, Amazon frogbit, or water sprite — to dim the lighting and provide cover. Mid-water stems and Cryptocoryne species along the back and sides for shelter and spawning sites. A dark substrate (black sand or fine ADA aquasoil) brings out the iridescent body color far better than light gravel.

Leave open swimming space across the front and middle so the school can dart and males can display in a clear field. Driftwood with attached Java fern or moss adds visual structure and releases mild tannins that match the wild habitat.

A Tank That Suits Them Well#

A small all-in-one like the Fluval Flex works well for threadfins. The 9 or 15-gallon versions hit the right footprint, the curved front shows off a planted scape, and the chambered filtration is straightforward to baffle for whisper-quiet flow.

Diet & Feeding#

Threadfins are micropredators with a mouth opening barely wider than a pinhead. Feeding them well is the single most demanding part of their care. Get the food size wrong and they slowly starve in a tank full of food.

Micro-Prey Requirements#

Build the daily diet around foods small enough to actually fit in the mouth. Micro pellets sized for fry (Hikari Micro Pellets, Bug Bites Spectrum Micro, Northfin Nano), crushed Omega One Color Flakes broken to powder, and live or frozen micro-organisms — daphnia, micro-worms, vinegar eels, freshly hatched baby brine shrimp.

Standard tropical flake fed whole is too large. If a pellet measures more than half a millimeter across, crush it between your fingers before it hits the water. Foods sold for adult tetras or larger community fish almost always need crushing for threadfins.

Feeding Frequency and Technique#

Feed two to three small meals a day. Each portion should be eaten in 60 to 90 seconds. Leftover food on the substrate two minutes after feeding means you fed too much.

Technique matters as much as food choice. Drop food on a still patch of surface — never directly into the filter output. Threadfins are slow, methodical feeders that pick prey out of the water column with deliberate strikes. Boisterous tank mates that race to the food source will outcompete them every time, and the threadfins will quietly fade.

Tank Mates & Compatibility#

The right tank mates are small, slow, peaceful, and uninterested in racing threadfins to the food bowl. The wrong ones are anything fast at feeding time or anything large enough to nip those trailing male filaments.

Suitable Nano Companions#

• Ember tetra (Hyphessobrycon amandae) — peaceful, similar size, calm feeders
• Chili rasbora (Boraras brigittae) and other Boraras species
• Pygmy corydoras (Corydoras pygmaeus, C. habrosus) for the substrate
• Otocinclus catfish for algae cleanup
• Other Pseudomugil rainbows like the forktail rainbowfish — same scale, same temperament
• Neocaridina shrimp (cherry, blue dream) and Amano shrimp — adults are safe; shrimplets may be picked at
• Nerite snails, mystery snails

Species to Avoid#

Skip any fin-nipper without exception — tiger barbs, serpae tetras, and rosy barbs will shred male threadfin filaments in a single afternoon. Avoid fast, boisterous feeders that outcompete them at the food source: zebra danios, larger barbs, big tetras. Stay away from anything large enough to view a 1.5-inch threadfin as a snack — angelfish, gouramis (other than the smallest sparkling gourami), all medium and large cichlids, and most catfish over 3 inches. Even bettas are a poor pairing because they harass the constantly darting school. The giant rainbows like the Boesemani rainbowfish are a size-mismatch — they will not eat threadfins, but they will outcompete them for food and intimidate them out of the open swimming column.

Breeding#

Threadfins are continuous egg-scatterers and breed readily once a group is conditioned. They are not the easiest rainbow to raise to adult size because the fry are tiny and need very small first foods, but spawning itself is straightforward.

Conditioning and Spawning Triggers#

Condition a group with daily live foods — baby brine shrimp and daphnia for 7 to 14 days. Bump the temperature up to 80-82 F. Males will intensify in color, flare their filaments constantly, and chase females through fine-leaved plants or spawning mops. Drop in a small clump of Java moss or a yarn spawning mop to give the females a defined egg-deposition site.

Raising Fry#

Eggs hatch in roughly 7 to 10 days depending on temperature. The challenge starts at hatch — the fry are microscopically small and cannot eat anything larger than infusoria for the first week to two weeks. Plan ahead. Start an infusoria culture or buy commercial fry powder before you remove the spawning mop to a separate rearing tank.

After 10 to 14 days the fry can take vinegar eels and freshly hatched baby brine shrimp. From that point forward they grow steadily and will reach a recognizable juvenile threadfin shape inside of two months. Without a dedicated rearing tank and prepared first foods, fry survival in the main tank is essentially zero — adults and other tank mates pick them off, and the food they need is rarely present in a typical community tank.

Common Health Issues#

Threadfins are hardy when water is stable, but their small body size means any disease moves fast. Catching problems in the first 24 hours is often the difference between a manageable issue and the loss of the entire school.

Velvet and Ich#

Both ich (Ichthyophthirius multifiliis) and velvet (Piscinoodinium pillulare) hit small-bodied fish hard. Ich appears as discrete white salt-grain spots on the body and fins; velvet as a fine gold-dust shimmer that you sometimes only catch in the right angle of light. Velvet is the more dangerous of the two and can wipe out a school in 48 hours.

Quarantine all new fish for 2 to 4 weeks before they enter the display tank. When treating for ich or velvet, use the heat method (raising temperature gradually to 86 F for 10 days) or copper-free options like ich-x first — copper-based medications kill any shrimp or invertebrates in the system and should only be used in a separate hospital tank.

Fin Damage and Stress#

The trailing filaments on adult males tear easily, and damaged filaments are slow to regrow. Most tears come from one of three causes: too much flow forcing the fish to fight current, decor with sharp edges (rough rockwork or freshly trimmed plant stems), or fin-nipping tank mates. Address all three before reaching for medication. Quarantine torn-fin fish in a calm hospital tank, keep water pristine, and the filaments will regrow over 4 to 8 weeks.

Where to Buy & What to Look For#

Threadfins are not a chain-store species. Petco and PetSmart almost never stock them. Specialty local fish stores, planted-tank-focused shops, and rainbowfish breeders are the realistic sources.

Finding Healthy Specimens at Your Local Fish Store#

A well-run LFS that caters to nano and planted-tank hobbyists will sometimes carry threadfins, and many will order them on request from their wholesaler. Ask. Inspect the tank carefully before buying — if a single fish in the system shows white spots, gold dust, or clamped fins, walk away from the entire group. The shipping and acclimation chain hits this species hard, and a stressed batch will keep dying for weeks after purchase.

If the store has no display males with developed filaments, the fish may be too young to confirm health, or they may have been shipped in too recently to recover. Either case is worth asking about before paying.

Online vs LFS Sourcing#

Online specialty rainbowfish breeders ship adult, color-developed fish that are usually higher quality than mass-imported wholesale stock. The trade-off is shipping stress — threadfins ship poorly compared to hardier nano fish, and even insulated packaging with heat packs sees occasional losses on multi-day routes. Door-to-door delivery from a hobbyist breeder a state or two away (1-day shipping) is often the best of both worlds.

If you have access to a hobbyist-driven local store, that is almost always the better starting point. You can inspect the fish in person, watch them eat, and acclimate them within an hour of purchase rather than after a 24-hour transit window.

Acclimation#

Drip acclimate over 60 to 90 minutes. Threadfins are sensitive to sudden parameter shifts, especially pH, hardness, and TDS swings. For step-by-step technique, see how to acclimate fish. Always quarantine new arrivals for 2 to 4 weeks in a separate tank with sponge filtration before adding them to an established community. This step alone prevents the majority of disease introductions.

Quick Reference#

• Tank size: 15 gallons minimum, 20 gallons recommended for a school of 8+
• Temperature: 73-82 F (23-28 C)
• pH: 6.0-7.5
• Hardness: 5-12 dGH
• School size: 6 minimum, 8-10 ideal, with a 2:1 female-to-male ratio to reduce male fin damage
• Diet: Micro pellets and crushed flake daily; baby brine shrimp, micro-worms, daphnia 2-3x weekly
• Tank mates: Ember tetras, chili rasboras, pygmy corys, otocinclus, dwarf shrimp, other Pseudomugil
• Avoid: Tiger barbs, serpae tetras, fast feeders, larger gouramis, cichlids, any fin-nipper
• Lifespan: 3-5 years
• Difficulty: Intermediate — water stability and micro-food sizing are the hard parts

Related Reading#

• Forktail rainbowfish care guide — the closest nano-rainbow comparison
• Boesemani rainbowfish care guide — see how the giant cousins compare
• Freshwater fish overview — broader context on choosing freshwater species
• Fluval Flex review — a strong all-in-one option for a planted threadfin scape