Photo: Wikimedia Commons contributor (CC BY-SA 2.5) — via Wikimedia Commons
Threadfin Rainbowfish (Iriatherina werneri)
A jewel-sized rainbowfish whose males trail impossibly long, ribbon-like fin extensions — one of the most ornate nano fish in the hobby.
Will it live with a Threadfin Rainbowfish?
We compare each fish against your threadfin rainbowfish on temperament, size, water parameters and swimming zone. Set your tank size and filter the results.
- African Dwarf Frog✅ CompatiblePeaceful · 4 cm · Medium care · 22–26 °C (72–79 °F)
- Both are peaceful, and their water overlaps around 24–26 °C — no size, zone or temperament conflicts.
- Keep Threadfin Rainbowfish in a shoal of 6+ or it gets stressed and nippy.
- Amapá Tetra✅ CompatiblePeaceful · 4 cm · Medium care · 24–27 °C (75–81 °F)
- Compatible on the things that matter: shared water near 24–27 °C, workable temperaments, and no predator-and-prey size gap.
- Keep Threadfin Rainbowfish in a shoal of 6+ or it gets stressed and nippy.
- Keep Amapá Tetra in a shoal of 6+ or it gets stressed and nippy.
- Black Phantom Tetra✅ CompatiblePeaceful · 4.5 cm · Easy care · 22–28 °C (72–82 °F)
- Both are peaceful, and their water overlaps around 24–28 °C — no size, zone or temperament conflicts.
- Keep Threadfin Rainbowfish in a shoal of 6+ or it gets stressed and nippy.
- Keep Black Phantom Tetra in a shoal of 6+ or it gets stressed and nippy.
- Blue Danio✅ CompatiblePeaceful · 4 cm · Easy care · 21–26 °C (70–79 °F)
- Both are peaceful; temperature, pH and hardness ranges all overlap and neither outsizes the other enough to be a threat.
- Keep Threadfin Rainbowfish in a shoal of 6+ or it gets stressed and nippy.
- Keep Blue Danio in a shoal of 6+ or it gets stressed and nippy.
- Blue Emperor Tetra✅ CompatiblePeaceful · 4 cm · Medium care · 23–27 °C (73–81 °F)
- Both are peaceful; temperature, pH and hardness ranges all overlap and neither outsizes the other enough to be a threat.
- Keep Threadfin Rainbowfish in a shoal of 6+ or it gets stressed and nippy.
- Keep Blue Emperor Tetra in a shoal of 6+ or it gets stressed and nippy.
- Cardinal Tetra✅ CompatiblePeaceful · 4 cm · Medium care · 23–27 °C (73–81 °F)
- Both are peaceful; temperature, pH and hardness ranges all overlap and neither outsizes the other enough to be a threat.
- Keep Threadfin Rainbowfish in a shoal of 6+ or it gets stressed and nippy.
- Keep Cardinal Tetra in a shoal of 6+ or it gets stressed and nippy.
- Emperor Tetra✅ CompatiblePeaceful · 4 cm · Easy care · 23–27 °C (73–81 °F)
- Both are peaceful, and their water overlaps around 24–27 °C — no size, zone or temperament conflicts.
- Keep Threadfin Rainbowfish in a shoal of 6+ or it gets stressed and nippy.
- Keep Emperor Tetra in a shoal of 6+ or it gets stressed and nippy.
- Flame Tetra✅ CompatiblePeaceful · 4 cm · Easy care · 22–28 °C (72–82 °F)
- Compatible on the things that matter: shared water near 24–28 °C, workable temperaments, and no predator-and-prey size gap.
- Keep Threadfin Rainbowfish in a shoal of 6+ or it gets stressed and nippy.
- Keep Flame Tetra in a shoal of 6+ or it gets stressed and nippy.
- Glowlight Tetra✅ CompatiblePeaceful · 4 cm · Easy care · 22–28 °C (72–82 °F)
- Both are peaceful, and their water overlaps around 24–28 °C — no size, zone or temperament conflicts.
- Keep Threadfin Rainbowfish in a shoal of 6+ or it gets stressed and nippy.
- Keep Glowlight Tetra in a shoal of 6+ or it gets stressed and nippy.
- Gold Tetra✅ CompatiblePeaceful · 4.5 cm · Easy care · 23–28 °C (73–82 °F)
- Both are peaceful; temperature, pH and hardness ranges all overlap and neither outsizes the other enough to be a threat.
- Keep Threadfin Rainbowfish in a shoal of 6+ or it gets stressed and nippy.
- Keep Gold Tetra in a shoal of 6+ or it gets stressed and nippy.
- Golden Dwarf Barb✅ CompatiblePeaceful · 4 cm · Easy care · 18–24 °C (64–75 °F)
- Both are peaceful; temperature, pH and hardness ranges all overlap and neither outsizes the other enough to be a threat.
- Keep Threadfin Rainbowfish in a shoal of 6+ or it gets stressed and nippy.
- Keep Golden Dwarf Barb in a shoal of 6+ or it gets stressed and nippy.
- Jelly Bean Tetra✅ CompatiblePeaceful · 4 cm · Medium care · 20–26 °C (68–79 °F)
- Both are peaceful; temperature, pH and hardness ranges all overlap and neither outsizes the other enough to be a threat.
- Keep Threadfin Rainbowfish in a shoal of 6+ or it gets stressed and nippy.
- Keep Jelly Bean Tetra in a shoal of 10+ or it gets stressed and nippy.
- Marbled Hatchetfish✅ CompatiblePeaceful · 4 cm · Medium care · 23–28 °C (73–82 °F)
- Compatible on the things that matter: shared water near 24–28 °C, workable temperaments, and no predator-and-prey size gap.
- Both favour the top of the tank — offer enough cover so they aren't always in each other's space.
- Keep Threadfin Rainbowfish in a shoal of 6+ or it gets stressed and nippy.
- Keep Marbled Hatchetfish in a shoal of 6+ or it gets stressed and nippy.
- Otocinclus✅ CompatiblePeaceful · 4 cm · Medium care · 21–26 °C (70–79 °F)
- Compatible on the things that matter: shared water near 24–26 °C, workable temperaments, and no predator-and-prey size gap.
- Keep Threadfin Rainbowfish in a shoal of 6+ or it gets stressed and nippy.
- Keep Otocinclus in a shoal of 6+ or it gets stressed and nippy.
- Phoenix Tetra✅ CompatiblePeaceful · 4 cm · Easy care · 23–28 °C (73–82 °F)
- Compatible on the things that matter: shared water near 24–28 °C, workable temperaments, and no predator-and-prey size gap.
- Keep Threadfin Rainbowfish in a shoal of 6+ or it gets stressed and nippy.
- Keep Phoenix Tetra in a shoal of 8+ or it gets stressed and nippy.
- Purple Tetra✅ CompatiblePeaceful · 4 cm · Medium care · 22–26 °C (72–79 °F)
- Both are peaceful; temperature, pH and hardness ranges all overlap and neither outsizes the other enough to be a threat.
- Keep Threadfin Rainbowfish in a shoal of 6+ or it gets stressed and nippy.
- Keep Purple Tetra in a shoal of 8+ or it gets stressed and nippy.
- Rainbow Emperor Tetra✅ CompatibleSemi-aggressive · 3.6 cm · Medium care · 22–28 °C (72–82 °F)
- Compatible on the things that matter: shared water near 24–28 °C, workable temperaments, and no predator-and-prey size gap.
- Keep Threadfin Rainbowfish in a shoal of 6+ or it gets stressed and nippy.
- Keep Rainbow Emperor Tetra in a shoal of 8+ or it gets stressed and nippy.
- Red Phantom Tetra✅ CompatiblePeaceful · 4 cm · Easy care · 22–28 °C (72–82 °F)
- Both are peaceful; temperature, pH and hardness ranges all overlap and neither outsizes the other enough to be a threat.
- Keep Threadfin Rainbowfish in a shoal of 6+ or it gets stressed and nippy.
- Keep Red Phantom Tetra in a shoal of 8+ or it gets stressed and nippy.
- Rosy Tetra✅ CompatiblePeaceful · 4 cm · Easy care · 22–28 °C (72–82 °F)
- Both are peaceful; temperature, pH and hardness ranges all overlap and neither outsizes the other enough to be a threat.
- Keep Threadfin Rainbowfish in a shoal of 6+ or it gets stressed and nippy.
- Keep Rosy Tetra in a shoal of 6+ or it gets stressed and nippy.
- Rummy Nose Rasbora✅ CompatiblePeaceful · 4 cm · Medium care · 20–26 °C (68–79 °F)
- Compatible on the things that matter: shared water near 24–26 °C, workable temperaments, and no predator-and-prey size gap.
- Keep Threadfin Rainbowfish in a shoal of 6+ or it gets stressed and nippy.
- Keep Rummy Nose Rasbora in a shoal of 8+ or it gets stressed and nippy.
- Sparkling Gourami✅ CompatiblePeaceful · 4 cm · Medium care · 24–28 °C (75–82 °F)
- Compatible on the things that matter: shared water near 24–28 °C, workable temperaments, and no predator-and-prey size gap.
- Both favour the top of the tank — offer enough cover so they aren't always in each other's space.
- Keep Threadfin Rainbowfish in a shoal of 6+ or it gets stressed and nippy.
- Strawberry Betta✅ CompatiblePeaceful · 4 cm · Medium care · 22–27 °C (72–81 °F)
- Both are peaceful, and their water overlaps around 24–27 °C — no size, zone or temperament conflicts.
- Keep Threadfin Rainbowfish in a shoal of 6+ or it gets stressed and nippy.
- Tiger Otocinclus✅ CompatiblePeaceful · 4 cm · Medium care · 22–27 °C (72–81 °F)
- Both are peaceful, and their water overlaps around 24–27 °C — no size, zone or temperament conflicts.
- Keep Threadfin Rainbowfish in a shoal of 6+ or it gets stressed and nippy.
- Keep Tiger Otocinclus in a shoal of 6+ or it gets stressed and nippy.
- Yellow Phantom Tetra✅ CompatiblePeaceful · 4 cm · Easy care · 23–27 °C (73–81 °F)
- Compatible on the things that matter: shared water near 24–27 °C, workable temperaments, and no predator-and-prey size gap.
- Keep Threadfin Rainbowfish in a shoal of 6+ or it gets stressed and nippy.
- Keep Yellow Phantom Tetra in a shoal of 6+ or it gets stressed and nippy.
- Black Darter Tetra⚠️ With cautionSemi-aggressive · 4 cm · Hard care · 21–28 °C (70–82 °F)
- Black Darter Tetra is semi-aggressive and may chase or nip the smaller Threadfin Rainbowfish — plant heavily and break up sight lines.
- Keep Threadfin Rainbowfish in a shoal of 6+ or it gets stressed and nippy.
- Black Ruby Barb⚠️ With cautionSemi-aggressive · 6 cm · Easy care · 22–26 °C (72–79 °F)
- Expect Black Ruby Barb to harass Threadfin Rainbowfish at times; give dense cover and watch them at feeding.
- Your 75 L tank is below the ~100 L this pairing really wants — crowding raises aggression.
- Keep Threadfin Rainbowfish in a shoal of 6+ or it gets stressed and nippy.
- Keep Black Ruby Barb in a shoal of 6+ or it gets stressed and nippy.
- Black Skirt Tetra⚠️ With cautionSemi-aggressive · 6 cm · Easy care · 20–26 °C (68–79 °F)
- Black Skirt Tetra is semi-aggressive and may chase or nip the smaller Threadfin Rainbowfish — plant heavily and break up sight lines.
- Keep Threadfin Rainbowfish in a shoal of 6+ or it gets stressed and nippy.
- Keep Black Skirt Tetra in a shoal of 6+ or it gets stressed and nippy.
- Cherry Shrimp⚠️ With cautionPeaceful · 3 cm · Easy care · 18–28 °C (64–82 °F)
- Threadfin Rainbowfish may eat Cherry Shrimp or pick off its shrimplets — a densely planted tank with moss gives them a fighting chance.
- Keep Threadfin Rainbowfish in a shoal of 6+ or it gets stressed and nippy.
- Keep Cherry Shrimp in a shoal of 10+ or it gets stressed and nippy.
- Desert Goby⚠️ With cautionSemi-aggressive · 6 cm · Easy care · 18–28 °C (64–82 °F)
- Expect Desert Goby to harass Threadfin Rainbowfish at times; give dense cover and watch them at feeding.
- Keep Threadfin Rainbowfish in a shoal of 6+ or it gets stressed and nippy.
- Dwarf Chain Loach⚠️ With cautionPeaceful · 6 cm · Medium care · 24–29 °C (75–84 °F)
- Your 75 L tank is below the ~80 L this pairing really wants — crowding raises aggression.
- Keep Threadfin Rainbowfish in a shoal of 6+ or it gets stressed and nippy.
- Keep Dwarf Chain Loach in a shoal of 6+ or it gets stressed and nippy.
- Ghost Shrimp⚠️ With cautionPeaceful · 4 cm · Easy care · 18–28 °C (64–82 °F)
- Threadfin Rainbowfish may eat Ghost Shrimp or pick off its shrimplets — a densely planted tank with moss gives them a fighting chance.
- Keep Threadfin Rainbowfish in a shoal of 6+ or it gets stressed and nippy.
- Keep Ghost Shrimp in a shoal of 6+ or it gets stressed and nippy.
- Humpbacked Tetra⚠️ With cautionSemi-aggressive · 5 cm · Hard care · 24–28 °C (75–82 °F)
- Expect Humpbacked Tetra to harass Threadfin Rainbowfish at times; give dense cover and watch them at feeding.
- Your 75 L tank is below the ~80 L this pairing really wants — crowding raises aggression.
- Keep Threadfin Rainbowfish in a shoal of 6+ or it gets stressed and nippy.
- Keep Humpbacked Tetra in a shoal of 8+ or it gets stressed and nippy.
- Morse Code Corydoras⚠️ With cautionPeaceful · 5 cm · Medium care · 23–26 °C (73–79 °F)
- Your 75 L tank is below the ~80 L this pairing really wants — crowding raises aggression.
- Keep Threadfin Rainbowfish in a shoal of 6+ or it gets stressed and nippy.
- Keep Morse Code Corydoras in a shoal of 6+ or it gets stressed and nippy.
- Serpae Tetra⚠️ With cautionSemi-aggressive · 4 cm · Easy care · 23–28 °C (73–82 °F)
- Serpae Tetra is semi-aggressive and may chase or nip the smaller Threadfin Rainbowfish — plant heavily and break up sight lines.
- Keep Threadfin Rainbowfish in a shoal of 6+ or it gets stressed and nippy.
- Keep Serpae Tetra in a shoal of 6+ or it gets stressed and nippy.
- Silvertip Tetra⚠️ With cautionSemi-aggressive · 5 cm · Easy care · 22–28 °C (72–82 °F)
- Silvertip Tetra and Threadfin Rainbowfish are close in size, but the semi-aggressive one tends to dominate — add threadfin rainbowfish in a group to spread the pressure.
- Keep Threadfin Rainbowfish in a shoal of 6+ or it gets stressed and nippy.
- Keep Silvertip Tetra in a shoal of 6+ or it gets stressed and nippy.
- Spotfin Betta⚠️ With cautionSemi-aggressive · 5 cm · Medium care · 23–28 °C (73–82 °F)
- Expect Spotfin Betta to harass Threadfin Rainbowfish at times; give dense cover and watch them at feeding.
- Keep Threadfin Rainbowfish in a shoal of 6+ or it gets stressed and nippy.
- Striped Red-Eye Puffer⚠️ With cautionAggressive · 5 cm · Hard care · 24–28 °C (75–82 °F)
- Striped Red-Eye Puffer is aggressive and may chase or nip the smaller Threadfin Rainbowfish — plant heavily and break up sight lines.
- Keep Threadfin Rainbowfish in a shoal of 6+ or it gets stressed and nippy.
- Tiger Badis⚠️ With cautionSemi-aggressive · 4 cm · Medium care · 22–24 °C (72–75 °F)
- Tiger Badis is semi-aggressive and may chase or nip the smaller Threadfin Rainbowfish — plant heavily and break up sight lines.
- Keep Threadfin Rainbowfish in a shoal of 6+ or it gets stressed and nippy.
- Tiger Shrimp⚠️ With cautionPeaceful · 3 cm · Hard care · 20–25 °C (68–77 °F)
- Threadfin Rainbowfish may eat Tiger Shrimp or pick off its shrimplets — a densely planted tank with moss gives them a fighting chance.
- Keep Threadfin Rainbowfish in a shoal of 6+ or it gets stressed and nippy.
- Keep Tiger Shrimp in a shoal of 6+ or it gets stressed and nippy.
- Wine Red Betta⚠️ With cautionAggressive · 5 cm · Medium care · 24–28 °C (75–82 °F)
- Wine Red Betta and Threadfin Rainbowfish are close in size, but the aggressive one tends to dominate — add threadfin rainbowfish in a group to spread the pressure.
- Keep Threadfin Rainbowfish in a shoal of 6+ or it gets stressed and nippy.
- Alligator Gar⛔ Not recommendedAggressive · 250 cm · Hard care · 24–28 °C (75–82 °F)
- Alligator Gar (250 cm) is big enough to swallow the 4 cm Threadfin Rainbowfish whole.
- Alligator Gar clearly outsizes Threadfin Rainbowfish and is aggressive; risky unless the tank is big and well-planted.
- Your 75 L tank is below the ~3785 L this pairing really wants — crowding raises aggression.
- Keep Threadfin Rainbowfish in a shoal of 6+ or it gets stressed and nippy.
- Clown Knifefish⛔ Not recommendedAggressive · 90 cm · Hard care · 24–28 °C (75–82 °F)
- Size gap is too large (90 vs 4 cm): Clown Knifefish will treat Threadfin Rainbowfish as food.
- Clown Knifefish clearly outsizes Threadfin Rainbowfish and is aggressive; risky unless the tank is big and well-planted.
- Your 75 L tank is below the ~750 L this pairing really wants — crowding raises aggression.
- Keep Threadfin Rainbowfish in a shoal of 6+ or it gets stressed and nippy.
- Fire Eel⛔ Not recommendedSemi-aggressive · 100 cm · Medium care · 24–28 °C (75–82 °F)
- Threadfin Rainbowfish is bite-sized to a 100 cm predatory fire eel — it will be eaten.
- Fire Eel is semi-aggressive and may chase or nip the smaller Threadfin Rainbowfish — plant heavily and break up sight lines.
- Your 75 L tank is below the ~380 L this pairing really wants — crowding raises aggression.
- Keep Threadfin Rainbowfish in a shoal of 6+ or it gets stressed and nippy.
- Koi⛔ Not recommendedPeaceful · 90 cm · Medium care · 4–28 °C (39–82 °F)
- Koi (90 cm) is big enough to swallow the 4 cm Threadfin Rainbowfish whole.
- Your 75 L tank is below the ~3800 L this pairing really wants — crowding raises aggression.
- Keep Threadfin Rainbowfish in a shoal of 6+ or it gets stressed and nippy.
- Redtail Catfish⛔ Not recommendedAggressive · 120 cm · Hard care · 24–27 °C (75–81 °F)
- Size gap is too large (120 vs 4 cm): Redtail Catfish will treat Threadfin Rainbowfish as food.
- Redtail Catfish is aggressive and may chase or nip the smaller Threadfin Rainbowfish — plant heavily and break up sight lines.
- Your 75 L tank is below the ~5700 L this pairing really wants — crowding raises aggression.
- Keep Threadfin Rainbowfish in a shoal of 6+ or it gets stressed and nippy.
- Spotted Gar⛔ Not recommendedAggressive · 90 cm · Hard care · 18–26 °C (64–79 °F)
- Size gap is too large (90 vs 4 cm): Spotted Gar will treat Threadfin Rainbowfish as food.
- Spotted Gar is aggressive and may chase or nip the smaller Threadfin Rainbowfish — plant heavily and break up sight lines.
- Your 75 L tank is below the ~600 L this pairing really wants — crowding raises aggression.
- Keep Threadfin Rainbowfish in a shoal of 6+ or it gets stressed and nippy.
- Wels Catfish⛔ Not recommendedAggressive · 300 cm · Hard care · 15–25 °C (59–77 °F)
- Threadfin Rainbowfish is bite-sized to a 300 cm predatory wels catfish — it will be eaten.
- Wels Catfish is aggressive and may chase or nip the smaller Threadfin Rainbowfish — plant heavily and break up sight lines.
- Your 75 L tank is below the ~20000 L this pairing really wants — crowding raises aggression.
- Keep Threadfin Rainbowfish in a shoal of 6+ or it gets stressed and nippy.
- Wolf Cichlid⛔ Not recommendedAggressive · 72 cm · Hard care · 24–30 °C (75–86 °F)
- Size gap is too large (72 vs 4 cm): Wolf Cichlid will treat Threadfin Rainbowfish as food.
- Wolf Cichlid clearly outsizes Threadfin Rainbowfish and is aggressive; risky unless the tank is big and well-planted.
- Your 75 L tank is below the ~760 L this pairing really wants — crowding raises aggression.
- Keep Threadfin Rainbowfish in a shoal of 6+ or it gets stressed and nippy.
Compatibility is computed from each species' care data — a strong starting point, not a guarantee. Individual temperament varies, so always introduce new fish slowly and watch them.
Threadfin Rainbowfish care specs
- Care level
- Medium
- Breeding
- Hard
- Max size
- 4 cm (1.6 in)
- Min tank size
- 60 L (15.9 gal)
- Temperature
- 24–28 °C (75–82 °F)
- pH
- 6–7.5
- Hardness
- 2–12 dGH
- Lifespan
- 3–5 years
- Diet
- Omnivore
- Swim level
- Top
- Group size
- 6+ (shoaling)
- Family
- Melanotaeniidae
- Origin
- New Guinea (southern lowlands) and northern Australia (Cape York Peninsula)
What is a Threadfin Rainbowfish?
The threadfin rainbowfish (Iriatherina werneri) is one of the smallest members of the family Melanotaeniidae, reaching only about 4 cm (1.6 in) at maturity. What it lacks in size it more than makes up for in spectacle: mature males grow extraordinary elongated filaments on the dorsal, anal and pelvic fins, giving them a lace-winged silhouette unlike any other freshwater fish commonly kept in the hobby. The body is translucent silver-blue; the extended fin rays are banded in black, red and yellow that catch the light brilliantly under a full-spectrum LED. Females are plainer with shorter, rounded fins, but remain elegant against a leafy planted backdrop.
Despite carrying the rainbowfish family name, threadfins sit in a category of their own — they are so much smaller and more delicate than classic rainbowfish like Melanotaenia boesemani or M. praecox that they are best grouped with nano fish in practice. They reward patient, experienced keepers with a display that is genuinely rare in freshwater fishkeeping. Care is rated Medium: water quality must be impeccable and feeding requires small-particle live or frozen foods, but the actual husbandry is not complicated once the basics are in place.
Where do Threadfin Rainbowfish come from?
Wild threadfin rainbowfish are found across two geographically connected regions: the southern lowland swamps and floodplains of New Guinea and the Cape York Peninsula of far-northern Australia. Both habitats share the same character — shallow, slow-moving or standing water thickly fringed with marginal vegetation, flooded meadow edges and papyrus swamps where current is negligible and the water is warm, soft and lightly tannin-stained.
This origin has direct implications for tank design. These fish evolved in still, sheltered water; strong filtration flow immediately stresses them and batters their delicate fin filaments. The soft, slightly acidic chemistry of their native swamps (pH 6.0–7.5, hardness 2–12 dGH) is also reflected in their preference for softer water at the lower end of the acceptable range. Virtually all fish in the aquarium trade are now captive-bred, which has improved overall hardiness somewhat, but the soft-water, low-flow preference is firmly fixed and must be respected.
What size tank do Threadfin Rainbowfish need?
A minimum of 60 litres (16 gallons) is appropriate for a group, and longer is always better than taller — threadfins are active surface and mid-water swimmers that appreciate horizontal swimming space. A 60–90 cm (24–36 in) footprint planted tank is ideal.
The tank must be densely planted. Threadfins use fine-leaved plants — java moss, hornwort, floating frogbit, water sprite, Microsorum — as cover and as a refuge from any perceived threat. Floating plants are particularly valuable because they diffuse the surface light, recreate the shaded conditions of their native habitat, and provide hiding spots that visibly reduce skittishness. Use a gentle, low-flow filter such as a sponge filter or an internal filter with a spray-bar diffuser. Aim for turnover no more than 3–4× the tank volume per hour. A secure lid is advisable since threadfins, like most small surface fish, can jump through small openings when startled.
What water parameters do Threadfin Rainbowfish need?
- Temperature: 24–28 °C (75–82 °F). A reliable heater is essential; these are strictly tropical fish.
- pH: 6.0–7.5. They tolerate a modest range but do best toward the slightly soft-acidic middle of that band.
- Hardness: 2–12 dGH. Soft water is preferable; very hard water above 12 dGH stresses them over time.
Water quality is the single biggest determinant of health in this species. Threadfins have very small body mass and delicate fins that show the damage from ammonia or nitrite far faster than a larger, hardier fish would. The tank must be fully cycled before any fish are introduced. Weekly partial water changes of 20–30% are a minimum; more frequent small changes keep nitrates low without large parameter swings. Avoid sudden temperature or pH shifts — even a 1–2 °C drop can suppress their immune system.
What do Threadfin Rainbowfish eat?
Threadfins are omnivores but are functionally micro-predators in the wild, picking tiny invertebrates and zooplankton from the water column and surface film. Their mouths are correspondingly small, and standard community flake or pellet food is simply too large for them to consume reliably.
Effective food choices include:
- Baby brine shrimp (newly hatched) — a staple and highly accepted
- Micro worms and Walter worms — easy to culture at home
- Daphnia (small-bodied) — stimulates natural feeding behaviour
- Cyclops and copepods — excellent nutritional profile
- Powdered or crushed flake as a supplement, not a primary food
Feed small amounts two to three times daily. Their slow, deliberate feeding style means they will be outcompeted at the food surface by faster fish, which is one reason tank-mate selection is so important. In a species-appropriate setup with gentle competition, threadfins quickly learn to associate the keeper’s presence with feeding and will gather at the front of the tank.
Are Threadfin Rainbowfish peaceful — and what are good tank mates?
Threadfins are unambiguously peaceful and utterly defenceless. Their long, flowing fin filaments are a standing invitation to fin-nipping species, and even mild semi-aggressive tank-mates can reduce them to ragged-finned, stressed shadows of their usual selves within days. The combination of their small body (4 cm / 1.6 in), peaceful temperament and delicate feeding requirements means tank-mate selection is among the most critical decisions in keeping them successfully.
Suitable companions share three traits: small size, completely peaceful disposition, and no fin-nipping tendency. Good choices include celestial pearl danios, chili rasboras, ember tetras, pygmy corydoras (Corydoras pygmaeus or C. hastatus), otocinclus, small peaceful dwarf shrimp (amano and neocaridina species) and other nano-friendly invertebrates. Avoid any barb, larger tetra, gourami or active cichlid regardless of how peaceful they are marketed — size and energy difference alone create feeding competition and fin harassment.
For a full filterable list of tested pairings, visit Threadfin Rainbowfish tank mates.
How do you tell male from female Threadfin Rainbowfish?
Sexual dimorphism in threadfin rainbowfish is pronounced and unmistakable in adult fish. Males develop the species’ namesake trait: dramatically elongated filaments extending from the dorsal, anal and pelvic fins. These filaments carry bold bands of black and red, and in peak condition they can trail to nearly the full body length of the fish, giving the male an extraordinary sail-like silhouette. Body colouration in males is also more vivid, with a stronger silver-blue iridescence.
Females are noticeably plainer — the fins remain short and rounded, body colour is softer, and there are no elongated filaments. Females are also fractionally smaller on average. Young fish of both sexes look similar until males begin developing fin extensions at around 8–10 weeks of age, making early sexing difficult. In a group of six or more fish, the ratio of males to females matters: keeping more females than males (or equal numbers) reduces male-to-male display stress, though males are not aggressive toward one another in the way that fighting fish are.
How do Threadfin Rainbowfish breed?
Threadfins are rated Hard to breed, partly because of their small size and delicate handling requirements, and partly because the fry are among the tiniest of any commonly kept rainbowfish, requiring infusoria-grade foods for the first week or two.
Spawning is an open-water, plant-scattering process. Males display intensely — spreading their elongated fins to full extent and manoeuvring alongside females — and pairs spawn among fine-leaved plants or moss, scattering small, adhesive eggs. Adults will eat eggs and fry if given the chance, so a dedicated breeding setup is advisable: a small tank (20–30 L is sufficient) with java moss or a spawning mop, aged soft water, gentle sponge filtration and a temperature toward the upper end of the range (26–28 °C / 79–82 °F) to stimulate spawning activity.
Fry hatch in 7–10 days depending on temperature and are extremely small. First foods must be appropriately tiny: infusoria, paramecia or commercial liquid fry food for the first 7–10 days, graduating to baby brine shrimp nauplii as the fry grow. Water quality management during the fry stage is critical — even low-level ammonia quickly kills small fry.
What diseases commonly affect Threadfin Rainbowfish?
The most common health problems in threadfins trace directly back to water quality and stress:
- Fin degradation / fin rot — the elongated filaments are the first indicator of poor water conditions; fraying, shortening or discolouration of the fin extensions is an early warning sign.
- Ich (white spot disease) — the classic white-spot outbreak, often triggered by a temperature drop or the introduction of an unquarantined fish.
- Velvet (Oodinium) — a fine golden or rust-coloured dusty sheen, particularly dangerous in small fish because it progresses rapidly.
- Bacterial infections — secondary to fin damage or chronic stress from incompatible tank-mates.
Prevention follows the same principles as for most small fish: maintain impeccable water quality, keep temperature stable within the 24–28 °C range, quarantine all new fish and plants for at least 2–3 weeks before introduction, and avoid overstocking. Threadfins are particularly sensitive to the stress of incompatible company, so a calm, appropriate tank-mate selection is itself a disease prevention measure.
Health note: medication dosing and diagnosis are beyond the scope of a care profile. If a fish shows signs of illness, confirm the symptoms against a reputable veterinary or fish-health reference before adding any treatment to the tank. Many medications at standard doses are harmful to small fish and invertebrates.
How long do Threadfin Rainbowfish live?
In good conditions, threadfin rainbowfish live 3–5 years. This is a reasonable lifespan for a fish of their size, and well-kept individuals often remain active and colourful throughout. The most common reasons for a shorter lifespan are cumulative stress from poor water quality, incompatible tank-mates causing fin damage and chronic stress, and nutritional deficiency from relying on foods that are too large for them to eat properly.
A threadfin rainbowfish in a mature, heavily planted, calm nano tank — fed daily on appropriately sized live and frozen foods and kept with equally peaceful companions — will reward the keeper with years of display. Males in full fin extension, catching the light in a densely planted tank, represent some of the finest viewing that freshwater nano fishkeeping has to offer.
Frequently asked questions
Can threadfin rainbowfish live with other fish?
Only with the gentlest, smallest tank-mates. Their flowing fin filaments invite nipping from anything remotely boisterous, and they cannot compete at feeding time with faster fish. Best companions are small rasboras, pygmy corydoras, small tetras, and peaceful micro-shrimp kept in a well-planted, calm tank.
Why are threadfin rainbowfish rated Medium care?
They are sensitive to water quality — ammonia or nitrite spikes damage their delicate fins quickly — and require very small live or frozen foods (baby brine shrimp, micro worms, daphnia) to thrive. A fully cycled, stable, well-planted tank is essential before adding them.
What you need to keep a threadfin rainbowfish
The baseline is a heated, filtered 60 L+ tank: a reliable heater to hold 24–28 °C (75–82 °F), a gentle filter that won't batter a threadfin rainbowfish in the current, and a tight-fitting lid. Cycle the tank fully before adding any fish.
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