Sundadanio is a genus of extremely small danionin fish endemic to Borneo and Sumatra, where its species inhabit blackwater peat-swamp forests and their acidic, tea-coloured streams. At 0.5–1 in, its members are among the smallest cyprinid relatives known, smaller even than most fish traded as "microrasbora." Intense, saturated neon colouration, electric blue, red, or violet depending on species, sets the genus apart visually, standing out sharply against the dark, tannin-stained water it favours in the wild.
The blue neon rasbora (Sundadanio axelrodi) is the genus's best-known aquarium export and a long-time staple of nano and blackwater biotope tanks, prized for shoaling brilliance in soft, acidic, tannin-rich water that mirrors its native peat swamps. Several more recently described Sundadanio species have followed it into specialist trade in smaller numbers. Conservation status is a serious concern here: two species are assessed as Endangered and three more as Vulnerable, consistent with a genus confined to peat-swamp habitat that continues to be lost to drainage, logging, and land conversion across Borneo and Sumatra.