Lake Kutubu Blue Rainbowfish

Melanotaenia lacustris — commonly sold as the Lake Kutubu Blue Rainbowfish — is widely regarded as one of the most spectacularly coloured rainbowfish in the hobby. Mature males develop a deep, laterally compressed body covered in scales of a uniform, intense metallic blue that is unlike the variable iridescence of other species. The fins are edged with pale blue-white, and the overall impression under aquarium light is that of a fish dipped in metallic paint.

This species is endemic to the Lake Kutubu region of Papua New Guinea and thrives in cool, clear, alkaline, and hard water — conditions that reflect its highland lake origin. It is one of the more demanding rainbowfish in terms of water quality, and soft, acidic, or warm water will quickly result in faded colouration and reduced vitality. Regular large water changes and powerful filtration are non-negotiable.

Melanotaenia lacustris is well-established in the hobby and is produced by specialist breeders worldwide. It spawns readily among fine-leaved plants when maintained in excellent condition, and small groups of one male to two females tend to produce the best spawning results.

Water: Hard, alkaline water essential; pH 7.5–8.5, temperature 23–27°C, GH 12–20, KH 6–12. Tank: Minimum 200 L; strong filtration, weekly 30% water changes; plants at margins with wide open swimming space. Feeding: High-quality flake and pellets as staple; live and frozen brine shrimp and daphnia for conditioning and colour. Breeding: Egg scatterer on fine plants; condition well with live foods; remove eggs to a rearing tank to maximise yield. Compatibility: Ideal with other large, peaceful rainbowfish; avoid warm-water or soft-water species that have conflicting requirements.