Melanotaenia gracilis is one of the more slender and streamlined members of the genus, originating from the southern lowland river systems of Papua New Guinea. Its elongated, laterally compressed body is covered in scales that shimmer with blue-green iridescence, while the dorsal and anal fins carry attractive orange tips that brighten conspicuously during male rivalry displays. The species is well-suited to aquariums with a moderate to brisk current, mimicking its natural riverine habitat.
This species is appreciably hardy once established and adapts well to a range of water conditions as long as extremes are avoided. It is rarely seen in mainstream fish shops but occasionally appears through specialist importers and hobbyist breeders. Kept in groups of six or more, males develop their most intense colouration and engage in energetic but harmless lateral display contests.
Melanotaenia gracilis breeds in the typical rainbowfish manner, scattering adhesive eggs among fine-leaved vegetation. Conditioning fish with live and frozen foods prior to breeding attempts improves egg yield. Fry are moderately small and can be raised on infusoria and brine shrimp nauplii.
