Threadfin rainbowfish

Quick facts

Adult size

4.5 cm

Lifespan

can live up to 4 years

Tank zone

middle

Temperament

peaceful

Difficulty

intermediate

Schooling

recommended 8+ (critical minimum 6, thrives at 12+)

Typically wild-caught

yes - acclimate slowly

Water parameters

Temperature

24–30°C

pH

5.5 to 7.5

Hardness

2 to 12 dGH

Tank requirements

Minimum volume

60 L

Minimum length

60 cm

Flow

low

Lighting

moderate

Substrate

any

Open swimming room

needed

Lid

required - jumper

Feeding

Diet: omnivore, feeds primarily at the middle.

Small mouths. Micro pellets, crushed flake, frozen baby brine shrimp, frozen cyclops, frozen daphnia, and live food (baby brine shrimp, daphnia, microworms). Feeds at the surface and mid-water. Not competitive at feeding time; easily outcompeted by faster fish. Target-feed if necessary. Two to three small feedings daily.

Compatibility

• Delicate nano fish (4–5 cm) that's easily overwhelmed by active or aggressive tankmates. Best with other small, peaceful species in a planted tank.
• Males develop extraordinary fin extensions (the "threadfins") that are used in display toward females and rival males. These extensions are fragile and will be nipped off by any fin-nipper in the tank.
• Good companions: ember tetras, pygmy corys, celestial pearl danios, shrimp, and other calm nano species. Avoid barbs, danios, and anything fast enough to outcompete them or nippy enough to target the fins.
• Males display constantly when females are present. A group of 3 males and 5-6 females in a planted 60-liter tank is one of the most underrated small-tank displays in the hobby.

Habitat

Native to swamps, streams, and flood plains in Papua New Guinea and Irian Jaya (western New Guinea). Found in shallow, warm, slow-moving water with dense aquatic vegetation. The species (Iriatherina werneri) was described in 1974. Males are small-bodied but develop dramatically elongated rays on the dorsal and anal fins that fan out during display like tiny flags. The body is olive to pale blue with an orange or red wash, and the extended fin rays are black and filamentous. Females are plainer with normal-length fins. The species is not a true Melanotaeniidae rainbowfish; it's the sole member of its genus and its placement within the rainbowfish family is based on shared anatomical features. Wild-caught and tank-bred specimens are both available. Tank-bred fish are increasingly common and generally hardier. The species prefers warm water (26–30°C), soft to moderately hard, and low flow.

Breeding

Egg scatterer that deposits eggs among fine-leaved plants and moss. Males display to females with spread fins, hovering and darting in a ritualized dance. The female deposits a few eggs daily among moss or spawning mops. Eggs are small and adhesive. Adults may eat eggs they find, so dense plant cover is important. Eggs hatch in 7-10 days (slower than most egg-scatterers). Fry are tiny and need infusoria or liquid fry food for the first week, then baby brine shrimp. Growth is slow. In a species-only planted tank with thick moss, fry appear spontaneously. Dedicated breeding uses removable spawning mops that are transferred to a hatching tank every few days.

Common problems

Fin damage from nipping tankmates or rough handling. The filamentous fin rays are the fish's main visual feature and take weeks to regrow when broken. Keep with non-nippy species only. Sensitivity to water quality: the species doesn't tolerate ammonia or nitrite spikes well. Ich is a risk during acclimation. Columnaris (bacterial) is the most common infection in suboptimal water quality. The warm-water requirement (26–30°C) limits compatible species choices. Short lifespan (3-4 years) means the colony needs to breed to sustain itself. The biggest non-medical issue is that people buy them for the dramatic photos and are disappointed when the fins aren't as developed as the pictures. Full fin extension in males takes 6+ months of maturity and good conditions.

Bioload

Bioload coefficient: 1.0 (tiny-bodied rainbow, very low waste).

Bioload coefficients are calibrated against the neon tetra as the anchor (1.0). See the methodology page for the formula and how each value was derived.

Further reading

• How to cycle a fish tank
• Stocking ideas by tank size