Gastromyzon is a genus of hillstream loaches in the family Balitoridae, endemic to the island of Borneo, with the atlas's records drawn from Malaysia, Indonesia, and Brunei. Its species are built for a specific niche: fast-flowing, well-oxygenated mountain and foothill streams with a hard substrate of rock and gravel. A dorsoventrally flattened body and broadly expanded, fleshy paired fins form a suction-like disc on the underside that lets these fish cling to rock faces even in strong current, while a ventral sucker-mouth is used to graze the algae and biofilm coating the rocks. The atlas covers 32 species ranging from about 1–4 in, and many show bold, species-specific banding or spotting across the back and fins, which has made the genus a magnet for careful taxonomic splitting over the years.
In the hobby, Gastromyzon is a cornerstone genus of the specialist hillstream-and-biotope aquarium movement rather than the general fishkeeping mainstream, kept for its striking flattened form and active, algae-grazing behaviour on open rockwork under strong current. Species such as the Blue Fin Hillstream Loach (G. aeroides), the Zebra Hillstream Loach (G. zebrinus), and the Gold Net Hillstream Loach (G. auronigrus) are among the more commonly traded names. Success depends on replicating cool, highly oxygenated, current-swept water rather than a standard community tank; they do poorly in stagnant or warm conditions. The IUCN picture is mixed but leans toward concern: alongside a sizeable Not Evaluated share, several species are rated Near Threatened or Vulnerable, reflecting Borneo's ongoing deforestation and stream degradation from logging, mining, and agricultural runoff.