Threadfin Rainbowfish
Iriatherina werneri
Min Tank Size
60L
Adult Size
3.5 cm
Lifespan
3 years
School Size
8+
About
Native to slow-moving, heavily vegetated waters of southern New Guinea and northern Australia, threadfin rainbowfish are genuinely tiny even by nano standards. Males are the showstoppers, carrying impossibly long extensions on their dorsal and anal fins that they fan and flutter constantly in displays toward rivals and potential mates. Females are plainer but still elegant, with a streamlined shape and a subtle iridescence. 5 cm, making them one of the smallest rainbowfish in the hobby.
Water conditions matter a lot with this species. They prefer soft to moderately hard water, slightly acidic to neutral pH, and warm temperatures. They're not particularly forgiving of ammonia or nitrite spikes, so a mature, stable tank is non-negotiable. A well-planted setup with floating plants and gentle filtration replicates their natural environment closely and brings out the best coloration in males.
Diet is where beginners often struggle. Threadfins have tiny mouths and need appropriately sized food. Micro worms, baby brine shrimp, and very small frozen cyclops or daphnia work well. Most will eventually accept high-quality micro pellets but don't count on it from day one.
Keep a group of at least eight, ideally with more females than males, to distribute male display behavior and reduce stress on any one individual. They're worth the extra effort. Few fish command attention the way a group of displaying males does in a planted nano tank, and seeing them in a setup built around their needs is genuinely impressive.
Water Parameters
Temperature
°CpH
GH
dGHKH
dKHSwimming Level
Flow Preference
Keeping multiple Threadfin Rainbowfish together
Threadfin Rainbowfish are shoaling fish and need company of their own kind. Keep a group of at least 8. Smaller groups leave them stressed, washed-out in color, and prone to hiding.
Compatibility
Compatibility is almost entirely about protecting threadfins from other fish rather than the other way around. Any fast, boisterous, or nippy species is a bad match. Tiger barbs, most danios, large tetras, and anything that might mistake threadlike fins for food are all off the list. Good companions include small peaceful species like ember tetras, chili rasboras, pygmy corydoras, and small livebearers if water parameters align. Dwarf shrimp make excellent tankmates. Avoid fish that compete aggressively at feeding time, since threadfins will simply lose access to food and slowly decline. A species-only setup or a carefully curated nano community works best.
Commonly kept with
Species this one is most often paired withCommonly tried but avoid
Often paired, but shouldn't beCare Notes
The most common mistake is underfeeding. Threadfins are easily overlooked at mealtimes, especially if kept with faster fish. Feed very small foods directly into the area where they're swimming and watch each individual eat. Water quality is the other major pitfall. These fish feel the effects of elevated nitrates before hardier species do, so weekly partial water changes are important even in a lightly stocked tank. Strong flow from filters stresses them and can physically impair their delicate fin extensions, so a sponge filter or a spray bar angled to minimize current is ideal.
Behavior & Aggression
Threadfins are about as peaceful as freshwater fish get. Males do display aggressively toward one another, flaring fins and circling in ritualized standoffs, but these interactions rarely escalate to actual nipping or injury. The only real concern is when tank space is very limited and males have no way to establish visual separation. More vegetation and line-of-sight breaks reduce male-to-male tension considerably. They pose zero threat to any tankmates.
Things to Know
- Easily outcompeted for food by faster or larger tankmates.
- Males may spar over females but rarely cause injury.
- Sensitive to poor water quality, do frequent small water changes.
- Requires very small live or frozen foods, may refuse dry food initially.
- Males have very delicate fins vulnerable to nipping.
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