Lacustris Rainbowfish

Melanotaenia lacustris is one of the most celebrated and recognisable rainbowfish in the entire hobby, routinely appearing in lists of the most beautiful freshwater fish. Its claim to fame is the extraordinary uniform metallic blue colouration of dominant males — not an iridescent shimmer that changes with angle, but a consistent, deep, vivid blue covering the entire body from snout to caudal peduncle. The fins are edged with pale blue-white, completing an almost monochromatic blue display that is utterly unique.

This flagship status comes with somewhat higher care demands. The species requires cool, hard, alkaline water reflecting its highland lake origin, and will lose its intense colour quickly in warm, soft, or acidic water. Regular substantial water changes and efficient filtration are essential. The aquarium should be spacious — minimum 200 litres — and groups of at least six are needed for natural social behaviour.

Melanotaenia lacustris is well-established in the hobby and available from specialist breeders worldwide. It breeds readily on fine-leaved plants and produces moderate-sized spawns. Fry develop colour at around six to eight months.

Water: Hard alkaline water non-negotiable; pH 7.5–8.5, temperature 23–27°C, GH 12–20; large weekly water changes. Tank: Minimum 200 L; strong filtration; wide-open swimming space; dense planting at sides. Feeding: High-quality flake and pellets; supplement with live and frozen invertebrates to maintain blue intensity. Breeding: Egg scatterer on fine plants; colour develops fully at 6–8 months in juveniles. Compatibility: Peaceful; pairs perfectly with other large, hard-water rainbowfish.