Threadfin Rainbowfish

Geographical Origin:

They inhabit the slow, warm swamps, lagoons, and heavily vegetated river margins among the floodplains of southern Papua New Guinea and the Cape York Peninsula in Australia. Areas where the water is often acidic, tannic, and still.

Unique Morphology:

They do not resemble any other Rainbowfish. The body is thin and laterally compressed. The first dorsal fin of the male is shaped like a pointed sail, the second dorsal and the anal fin erupt into extremely long black filaments that extend well beyond the tail.

Tank Setup:

Treat them as the Rainbowfish equivalent of wild Discus. They detest blinding light and fast movement. The tank (60-80 cm / 24-30 inches) must be densely planted and have an almost imperceptible surface current. Include tangled roots, oak or Catappa leaves on the bottom, and a canopy of floating plants (which will also provide refuge for aquatic infusoria, on which they feed). Any fin-nipping tankmate will reduce their elegant fins to shreds, killing them via bacterial infections.

Feeding Challenge (Micro-predators):

Most *Iriatherina* starve in normal aquariums because the aquarist does not realize that the fish physically cannot swallow normal pellets. They have literally microscopic mouths and throats for their size (adapted to catch suspended unicellular algae and protozoa). The only guarantee of success is the daily supply of suspended live micro-food or very fine powders and flakes crushed to the level of impalpable dust. Keep them with colonies of small organisms in well-matured tanks.

Sexual Dimorphism:

Extreme. Females are a dull honey-grey, totally lacking the dark filamentous extensions on the fins, and noticeably smaller and stockier. They are, however, the social engine: without them, the males will never show the fabulous jerky fin-opening behaviors.