Threadfin Rainbowfish

The threadfin rainbowfish is recognized by the fine, black, thread-like rays that extend from the males' second dorsal fin and anal fin to beyond the tail. A first dorsal fin that opens like a fan and a lyre-shaped tail with red edges complete the look. It grows to only 4–5 cm.

It is the only species in its genus and belongs to the rainbowfish family (Melanotaeniidae). Because it is a peaceful, shy and slow-eating fish, it should be kept with calm small fish, not with active or fin-nipping species.

It lives in the lowlands of northern Australia and southern New Guinea, in densely vegetated swamps, ponds and along the shallow edges of slow-flowing streams. The water is warm, soft and usually slightly acidic; the fish move in loose groups among aquatic plants.

• Volume: at least 60 liters and 60 cm in length for a group of 6–10.
• Water: 25–27°C, soft to moderately hard, slightly acidic to neutral water. Stable values are important.
• Plants: Dense plants and floating plants give it confidence; leave a small open area in the middle.
• Flow: Should be weak; long-finned males struggle in strong flow.
• Lid: It can jump; use a lid.

Omnivorous, but its mouth is very small. It is fed micro granules, finely crushed flakes, newly hatched brine shrimp, cyclops and daphnia. Because it eats slowly, it can go hungry if kept with fast fish; feed small amounts several times a day and make sure everyone eats.

Peaceful and somewhat shy; it moves near the water surface and in the middle layer. Males constantly spread their fins and display to each other and to the females; this dance is the species' most beautiful behavior. The males' long rays become a target for fin-nipping fish.

Good companions: blue-eye species, small rasboras, dwarf corydoras, otocinclus, Neocaridina shrimp, nerite snails.

Avoid: fin-nipping barbs and tetras (e.g. tiger barbs), fast and large rainbowfish, aggressive or large fish.

At least 6, ideally 10 or more individuals; a female-heavy group (2 females per male) is best. Having several males keeps the display dance going constantly. When kept in small numbers it becomes skittish and hides.

A continuous-spawning egg scatterer. Pairs lay a small number of eggs every day on fine-leaved plants, moss or a wool spawning mop. Because the parents may eat the eggs, they should be moved to a separate container. The fry hatch in about 7–10 days; they are very small and need to be fed infusoria or green water in the first days, moving on to brine shrimp about a week later.

Hybridization: Since it is the only species in its genus, the risk of hybridizing with other species is low; however, different locality forms of Australian and New Guinea origin should not be mixed.

• Starvation: the most common problem when kept with fast fish is underfeeding.
• Fin damage: the long rays can get caught on decor or by other fish.
• Velvet and white spot: can appear after transport; quarantine new fish.
• Its lifespan is relatively short; in good conditions it lives a few years.

Males have a large, fan-shaped first dorsal fin; long black threads extend from their second dorsal and anal fins, and their tails have red edges. Females are plain silvery-yellowish; their fins are short, rounded and colorless.