Neon Rasbora

The Neon Rasbora (Sundadanio axelrodi) is widely regarded as one of the most visually spectacular micro fish in the freshwater hobby. Native to the blackwater peat swamp forests of Borneo, this tiny species reaches only about 2 cm in length but makes an extraordinary visual impact when kept in groups, thanks to its intensely iridescent blue-green body that seems to glow from within under aquarium lighting. The coloration shifts depending on viewing angle and light conditions, creating a dynamic, jewel-like display. Taxonomically, Sundadanio axelrodi belongs to the family Danionidae and is closely related to rasboras, though classified in its own genus. In the wild it inhabits extremely soft, acidic, tannin-rich waters with minimal hardness — conditions that must be replicated closely in captivity for the fish to thrive and maintain its vibrant coloration. Water chemistry that deviates significantly from these parameters will cause the fish to appear dull and become prone to illness. The ideal setup is a blackwater nano aquarium with peat or botanical filtration, dark substrate, Indian almond leaves, fine-leaved plants, and dim lighting. The Neon Rasbora is peaceful and compatible with other micro species, dwarf shrimp, and small invertebrates. It should only be housed with tankmates of similar temperament and size. Its tiny mouth size limits food options to micro-sized particles.

Tank Setup: A minimum of 20 liters is recommended, though 30–40 liters allows for a larger, more visually impressive group. Use a dark substrate (black sand is ideal), driftwood branches, and Indian almond leaves to create a proper blackwater biotope. Fine-leaved plants such as Java moss and Hemianthus provide cover. Floating plants help soften light and reduce surface agitation.

Water Parameters: Extremely soft, acidic water is non-negotiable. Target pH 4.0–6.5, GH 0–4, KH 0–2, and temperature 22–28°C. Use RO water with minimal remineralization or collected rainwater. Peat filtration and blackwater botanicals are strongly recommended. Consistent parameters matter more than hitting exact targets.

Filtration: A sponge filter is the best choice for this species. Water movement should be gentle and slow, mimicking the still or barely moving forest streams of their native habitat.

Feeding: Their small mouths require micro-sized foods. Offer baby brine shrimp, micro worms, vinegar eels, daphnia, and finely crushed high-quality flake. Feed twice daily in very small amounts to avoid polluting the water.

Social Behavior: Keep in groups of at least 8, ideally 12–20 for a spectacular display effect. The schooling behavior and iridescence in large groups is one of the highlights of nano fishkeeping. Compatible with dwarf shrimp and other Boraras species.

Health: Very sensitive to water quality and hardness. Do not use any copper-based treatments. Sudden parameter shifts can be fatal. Quarantine new fish thoroughly before introducing them to an established nano setup.