Threadfin Rainbowfish ~ Iriatherina werneri

The Threadfin Rainbowfish is a delicate, ethereal species admired for its slender body, long trailing fins, and elegant, fluttering swimming style. Native to the shallow streams, swamps, and flooded grasslands of northern Australia and New Guinea, this rainbowfish is distinguished by its extended dorsal and anal fin rays, which resemble fine threads that ripple with every movement. Males become especially vibrant, displaying shimmering silver, gold, and pastel hues as they perform courtship dances. Their peaceful nature and graceful motion make them a captivating highlight in well-planted nano to mid-sized aquariums.

• Size: 1.5-2 inches

• Lifespan: 3-5 years

• Temperament: Peaceful, timid

• Group/Solitary: Best kept in groups of 6+, with multiple males to encourage displays

• Food: Micro-pellets, crushed flakes, baby brine shrimp, daphnia, cyclops, other small live or frozen foods

• Water Requirements: 75-82°F, pH 6.0-7.5, soft to moderately soft water

Threadfin Rainbowfish thrive in mature, densely planted aquariums with gentle flow and plenty of open space near the surface for their elaborate fin displays. Because of their small mouths and delicate build, they require fine-textured foods and benefit greatly from live or frozen micro-prey. These fish are shy by nature and should be housed with peaceful, non-nippy tank mates to avoid damage to their elongated fins. A calm environment, stable water parameters, and high water clarity are essential to keeping them confident and displaying their best colors. Male threadfins are especially impressive when kept together, as they engage in constant, harmless displays that enhance their visual appeal.

In the wild, Threadfin Rainbowfish inhabit shallow, warm waters filled with submerged grasses, fallen branches, and floating plants that diffuse sunlight and create quiet, sheltered zones. These environments foster abundant microfauna that support their natural feeding habits. Their elongated fin rays evolved in part for courtship, allowing males to signal and posture with precision in the still, clear margins of their native waters.

Fun Fact: During courtship, male Threadfin Rainbowfish spread their long fins into full fans and shimmer in the light, performing a dance so elaborate it can appear as though the fish is hovering in place.