Threadfin Rainbowfish

Geographic Origin and Biotope: Native to grassy swamps and floodplains of northern Australia and Papua New Guinea. Inhabits extremely slow or stagnant waters among dense aquatic plant forests.

Taxonomy and Morphology: A Melanotaeniidae (max 5 cm) with spectacular anatomy. The second dorsal and anal fins extend into extremely long black filaments in adult males, sometimes exceeding total body length.

Social Behavior: Males spend much of the day in chromatic display competitions, vibrating their filaments like insect antennae. Extremely peaceful but incredibly delicate: competition stress with faster species causes feeding cessation.

Coloration and Sexual Dimorphism: Extreme dimorphism. Mature males have silvery bodies with amber reflections and characteristic long black silk-like filaments. Females are smaller with normal fins and uniform olive-silver coloration.

Tank Setup: Dedicated 60+ liter aquarium, densely planted with fine-leaved plants and ample floating cover. Current must be nearly nonexistent: fragile filaments are torn off by pump turbulence.

Feeding: Tiny upturned mouth. Needs microscopic foods: artemia nauplii, cyclops, live daphnia, microworms. Standard flakes are ignored or too large. Poor nutrition is the second cause of death after filament mutilation.

Water Quality: Tolerates wide pH (5.5-7.5) but is lethally intolerant of organic pollution. Zero ammonia and nitrite. Biweekly 20% water changes at identical temperature.

Compatibility: THE CRITICAL POINT. Almost NO fish is a safe companion. Barbs, Tetras, Guppies, Corydoras will all eventually bite the long filaments. Only safe with Pseudomugil, Boraras, or small shrimp.

Reproduction: Daily egg scatterers. Filamentous eggs attach to mosses. Fry are microscopic and need infusoria, then artemia nauplii.

Risks: Filament mutilation is the main tragedy. Once torn, they never regrow to original length. The mutilated fish loses display ability and falls into chronic depression. Columnaris attacks damaged filament tips.