Neon Blue Rasbora

The neon blue rasbora is a tiny, spectacularly coloured fish from the blackwater habitats of the Greater Sunda Islands in Indonesia. Wild Sundadanio axelrodi is known from the peat swamp forests of Bintan Island, Indonesia. Males display a striking iridescent blue-green stripe along the flank over a coppery body, with bright red colouration in the dorsal, anal, and caudal fins. Females are more subdued with a silver-white body and yellow fins. Adults reach only 2.5 cm (1 inch), ranking them among the smallest of aquarium cypriniforms and ideal candidates for the nano-aquarium.

This species inhabits pristine peat swamp forests and blackwater streams with extremely soft, acidic water stained dark tea-brown by decaying organic matter. Replicating these conditions in the aquarium is essential for long-term health — the water should be soft (GH < 5 dGH), very acidic (pH 4.0-6.5), and enriched with tannins from alder cones, Indian almond leaves, or commercial blackwater extract. A group of 8-12 individuals requires a tank of at least 30 litres with dense planting, floating vegetation, and very dim lighting. They are extremely sensitive to water quality fluctuations and must only be introduced to mature, stable aquaria.

Water parameters: 23-26°C, pH 4.0-6.5. The diet must reflect their micropredatory nature — they will accept finely crushed flake foods and micro-granules but require regular live and frozen offerings of Artemia nauplii, Daphnia, microworms, and Cyclops. A varied diet is critical for maintaining their intense colouration. Due to their specialised water chemistry requirements, sensitivity, and diminutive size, the neon blue rasbora is best suited to experienced aquarists with dedicated softwater, blackwater setups. When kept correctly, this tiny gem transforms into a shimmering jewel that rivals any freshwater fish in intensity of colour.