Sundadanio axelrodi Blue

Sundadanio axelrodi is a minuscule danionid from the blackwater peat swamp streams of Borneo, and the blue colour form is among the most vivid of its several regionally distinct colour morphs. Under aquarium lighting the fish appears to emit a neon blue glow from its flanks, an optical effect produced by structural iridophores in the skin rather than true pigmentation.

At just 2 cm in length it is one of the smallest fish in the hobby, requiring a dedicated nano setup with very gentle filtration, soft and acidic water replicating its blackwater origins, and a dark substrate to maximise the visual impact of its colouration. A group of fifteen or more creates a spectacular moving cloud of neon blue in a carefully aquascaped blackwater tank.

Sundadanio axelrodi is not a beginner fish — its specific water chemistry requirements, tiny size, and sensitivity to water quality place it firmly in the intermediate category. It is best kept as a species-focused display or alongside equally small and soft-water-tolerant tankmates.

Water: Temperature 22–28°C, pH 4.0–6.5, GH below 6; soft, acidic blackwater conditions essential; use RO water or peat filtration. Tank: Minimum 20 L nano; dark substrate; blackwater tannins from driftwood or Indian almond leaves; very gentle sponge filtration. Feeding: Micro-pellets, rotifers, cyclops, and baby brine shrimp; mouth is extremely small so food size is critical. Breeding: Egg-scatterer; rarely bred in captivity; blackwater conditions and live foods may encourage spawning attempts. Compatibility: Species tank or with equally tiny blackwater fish; avoid all large or boisterous tankmates that will outcompete or stress this delicate species.