Threadfin rainbowfish
Iriatherina werneri
Threadfin rainbowfish exhibit a slender body with vibrant iridescent blue and green hues, marked by long, delicate thread-like fin extensions.
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About the Threadfin rainbowfish
This is the little rainbowfish with the ridiculous, delicate streamers - especially on the males, who love to posture and "dance" at each other in a calm planted tank. Keep them in a real group and they get way braver, cruising the top/midwater under floaters and showing off those thread-like fins. They are peaceful, but they really hate fast flow and rough tankmates that shred fins or outcompete them at feeding time.
Also known as
Quick Facts
Size
4-5 cm
Temperament
Peaceful
Difficulty
Intermediate
Min Tank Size
20 gallons
Lifespan
3-5 years
Origin
Australia and New Guinea
Diet
Omnivore/micro-predator - tiny foods like micro pellets, crushed flakes, baby brine shrimp, daphnia
Care Notes
- Go bigger than you think: a 20 gallon long (or similar footprint) with plants and some open swimming room keeps them from getting jumpy and beat up.
- Keep the water on the soft-to-moderate side and slightly acidic to neutral (about pH 6.0-7.5; ~2-12 dGH) with stable temps around 24-29°C; they dislike abrupt swings and strong current.
- They are timid at feeding time, so use small foods that hang in the water column: baby brine shrimp, daphnia, cyclops, and fine micro-pellets; feed small amounts 1-2 times a day.
- Keep them in a group (8+ is great) with more females than males, otherwise one flashy male will hassle the others and fray fins.
- Skip fin nippers and pushy fish (most barbs, tiger anything, many larger tetras, aggressive gouramis); they do best with calm tankmates like small rasboras, ember tetras, otos, and peaceful shrimp.
- Give them gentle flow and clean water, but not a blasting current - their long fins get torn up and they stop showing off.
- If you want babies, add a fine-leaf plant or spawning mop and pull the eggs every few days; adults will snack on fry if they can find them.
- Watch for skinny fish and clamped fins after adding them - they ship poorly and can come in with internal parasites, so quarantine and treat early if they keep losing weight while eating.
Compatibility
Good Tankmates
- Small, calm tetras (e.g., ember tetras, green neon tetras). Choose species that won't outcompete them at feeding time.
- Peaceful rasboras - chili rasboras, harlequins, lambchops. Similar vibe and they do not mess with the threadfins' long fins.
- Corydoras (the smaller, chill types) - pygmy, habrosus, panda. They stick to the bottom and keep things lively without stressing the threadfins.
- Otocinclus and other gentle algae crew - otos in a little group are perfect. They are calm and do not compete hard for food.
- Small, peaceful gouramis - sparkling gourami is a classic match. They are not usually fin-nippy and they like similar planted, calm setups.
- Non-nippy livebearers like endlers (the smaller strains) - they can work if your endlers are not hyper and the tank is planted so threadfins can chill out.
Avoid
- Fin nippers - tiger barbs, serpae tetras, many larger danios. Those long threadfins are basically a chew toy for anything nippy.
- Betta splendens and other territorial centerpiece fish - some bettas ignore them, but a lot see the fluttery fins and either harass them or get stressed and start trouble.
- Big or boisterous fish - larger rainbows, most cichlids, anything that hogs the middle and slams food. Threadfins get outcompeted and go timid fast.
- Semi-aggressive gouramis - dwarf gourami can be a coin flip, and three-spot/opaline types are usually too pushy. Threadfins do best with zero drama.
Where they come from
Threadfin rainbows (Iriatherina werneri) come from northern Australia and southern New Guinea - think warm, weedy backwaters and slow creeks with lots of plants and tannins. They look delicate because they kind of are, but once settled they are way tougher than their reputation.
Setting up their tank
Give them a calm, planted tank and they act like a completely different fish. They do not love chaos: strong current, aggressive tankmates, and constant glass-banging will keep them washed out and hiding.
Tank size wise, a 20 long works nicely for a proper group, but bigger is always easier if you want them colored up and displaying. Height helps too because males like to posture in open water above the plants.
- Group size: 8-12+ (you will see way better behavior in a real school)
- Filtration: gentle flow; sponge filters or a baffled HOB are your friend
- Plants: dense fine-leaf stuff (java moss, guppy grass, cabomba, myriophyllum), plus floating cover
- Hardscape: keep lots of open swimming room, but break sightlines with plant clumps
- Lighting: moderate; too bright with no cover makes them skittish
If you want one simple upgrade: add floating plants and dim the tank a bit. I have watched threadfins go from pale and jumpy to relaxed and showing off in a day.
Water wise, they are pretty flexible as long as it is clean and stable. Slightly acidic to neutral is common, but I have kept them in neutral to mildly hard water without drama. What they hate is dirty water and sudden swings.
- Temperature: 75-80F (24-27C) is a comfortable range
- pH: roughly 6.0-7.5 works for most people
- Hardness: soft to moderately hard is fine if stable
- Maintenance: regular water changes beat chasing numbers
They can jump, especially when spooked. Use a lid, cover gaps around airlines, and do not leave the waterline right at the rim.
What to feed them
Threadfins have tiny mouths and they are picky about particle size. If you feed big flakes or chunky pellets, a lot will just get ignored and rot. Small foods, offered more than once a day, gets the best growth and finnage.
- Staples: micro pellets, crushed high-quality flake, small granules
- Frozen: cyclops, baby brine shrimp, daphnia, finely chopped bloodworms (not huge pieces)
- Live (if you can): baby brine shrimp, microworms, grindal worms, moina
I like to feed them like I feed picky tetras: two small meals a day, and at least one of those is tiny frozen or live. You will notice the males display more when they are getting meaty foods.
How they behave and who they get along with
They are peaceful, a little shy, and honestly kind of polite. Males will flare and dance at each other, but it is more like competitive posing than real fighting. The long fins are the whole point of the fish, so you have to protect them from nippers.
- Good tankmates: small rasboras, ember tetras, pencilfish, small peaceful Corydoras, Otocinclus, small peaceful shrimp (adults usually fine)
- Usually fine: other gentle rainbowfish species that are not hyperactive (watch for competition at feeding time)
- Avoid: tiger barbs, most fin-nipping tetras, aggressive gouramis/cichlids, big fast feeders that bulldoze meals
They look best with more females than males, but you still want a few males so you get the fin displays. In a group of 10, something like 3-4 males and the rest females works well.
One behavior thing that surprises people: they do not love being the only midwater fish. If they are the main target of attention in the tank, they stay nervous. A calm dither group (like small rasboras) can help them relax.
Breeding tips
They are egg scatterers and will spawn pretty readily once they settle in. You will see the male display with those threadlike fins and lead the female into fine plants or moss. The catch is that adults will snack on eggs and tiny fry, so you need a plan.
- Easiest method: a separate small breeding tank with a big wad of java moss or spawning mop
- Conditioning: feed live/frozen small foods for a week or two
- Spawning: they lay a few eggs each day rather than one big dump
- Egg handling: move the mop/moss to a grow-out container every couple days
- Fry food: infusoria/rotifers first, then microworms and baby brine shrimp as soon as they can take it
If you do not want a whole breeding setup, you can still get a few survivors by keeping the tank heavily planted with moss and feeding the adults well. But if you want numbers, pull the eggs or pull the parents.
Common problems to watch for
Most threadfin issues come from stress: too much flow, too much aggression, not enough cover, or water quality sliding. They are not the fish I use to test whether a tank is stable.
- Fin damage: usually from nippers or rough netting; use a soft net and slow movements
- Not eating: food is too big or tankmates are outcompeting them
- Wasting away: often internal parasites in new imports; quarantine helps a lot
- Shimmying or clamped fins: check ammonia/nitrite first, then look at temperature swings and stress
- Sudden losses after a water change: they did not like the change in temp/TDS; match new water more closely
Go gentle with acclimation and big parameter shifts. If your tap water is very different from the shop water, drip acclimation and smaller, more frequent water changes can save you a lot of heartbreak.
Last thing: buy the best-looking group you can and do not mix obviously skinny, pinched-belly fish into your main tank. A calm quarantine with easy foods (baby brine, cyclops, crushed flake) is the fastest way to get them eating and filling out.
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