Threadfin Rainbowfish Identification Guide

Key identification features

• Very small and slender, typically 1.5-2 inches (4-5 cm)
• Semi-transparent yellowish body with a fine dark stripe along the midline
• Greatly elongated, thread-like rays trailing from the second dorsal, anal, and tail fins, especially pronounced in males
• Tail lobes drawn out into fine points rather than an evenly forked shape
• Delicate, wispy overall silhouette compared to other rainbowfish
• First dorsal fin reduced to just a few short spines, making the second dorsal appear dominant

Common look-alikes

• Celebes Rainbowfish: also has extended fin rays but is larger, more robust, and shows a blue-green sheen
• Neon and Dwarf Neon Rainbowfish: both lack any filamentous fin extensions and are more solidly colored
• Forktail Rainbowfish: has a genuinely forked, pointed tail but no long trailing fin threads

Where you'll see one

Found in still, densely vegetated swamps and slow streams across southern New Guinea and far northern Australia, usually hovering near cover in loose groups. It rarely strays into open, fast-flowing water.