Neon Red Rasbora

Also known as the Red Axelrodi Rasbora, Sundadanio rubellus is a very small Southeast Asian cyprinid native to western Borneo. It is documented from peat swamp forest habitats in West Kalimantan, Indonesia, particularly within lowland blackwater systems associated with the lower Kapuas River drainage. These habitats consist of shallow swamp channels, forest pools, and slow-moving blackwater streams with extremely low light levels, dense leaf litter, submerged wood, and heavy organic accumulation. Water in these environments is typically darkly stained, very soft, highly acidic, and extremely low in dissolved minerals.

Adult Sundadanio rubellus is tiny, usually reaching only about 0.75 to 1 inch in total length. The body is slender and semi-translucent, marked by a vivid red to reddish-orange horizontal stripe running along the flank. In well-conditioned fish, especially mature males, this stripe appears intensely saturated and almost glowing against the dark body. Subtle iridescent blue or green highlights may be visible near the head and fins under subdued lighting. Females are slightly fuller-bodied and generally less intensely colored, though they still show the characteristic red lateral stripe.

A tank of at least 10 to 15 gallons is appropriate for a proper group, with larger volumes providing greater stability and improved long-term success. Sundadanio rubellus is a schooling species and should be kept in groups of ten or more to reduce stress and promote natural behavior. The aquarium should be heavily structured with leaf litter, fine branches, botanicals, and floating plants to diffuse light and create a shaded, enclosed environment. Open swimming space should remain through the middle of the tank, but overall lighting should be kept low.

Water conditions are critical to success. Sundadanio rubellus requires very soft, acidic water, ideally with a pH below 6.0 and minimal hardness. Temperatures between 74 and 80 degrees Fahrenheit are appropriate. Stability is essential, as this species is highly sensitive to rapid changes in water chemistry. The use of leaf litter, seed pods, and natural wood helps maintain suitable blackwater conditions and supports natural coloration and behavior.

Feeding Sundadanio rubellus requires extremely small foods. In nature it feeds on tiny insects, microcrustaceans, and other minute organisms suspended in the water column. In the aquarium it accepts micro-pellets, finely crushed flakes, and frozen foods such as baby brine shrimp, cyclops, and daphnia. Food particle size is especially important due to the very small mouth.

Temperament is peaceful, shy, and non-aggressive. Sundadanio rubellus should only be kept with other very small, gentle species that tolerate similar blackwater conditions. Suitable tankmates include other Sundadanio species, small Boraras, and delicate blackwater-adapted fish. Larger or more active fish will cause stress and interfere with feeding.

Breeding is possible but challenging and usually occurs only in very soft, acidic water under low-light conditions. Eggs are scattered among fine plants or leaf litter, and adults do not provide parental care. Eggs and fry must be protected if breeding is desired.

Sundadanio rubellus is a delicate, jewel-like schooling fish best suited for dedicated blackwater aquariums. Its tiny size, intense red coloration, and subtle behavior make it an exceptional choice for experienced aquarists who can provide stable, soft, acidic water and a calm, naturalistic environment.