Erromyzon is a genus of hillstream loaches (family Balitoridae, sometimes treated within Gastromyzontidae) native to fast-flowing streams in southern China and Vietnam. At 1.5–2.5 in, its species share the classic hillstream-loach body plan: a flattened head and body, and enlarged, fan-like paired fins that act as a suction disc, letting the fish cling to smooth rock faces in strong current rather than being swept downstream. They typically graze algae and biofilm off rock surfaces in clear, oxygen-rich, fast-moving water — a very different niche from the soft-substrate loaches most aquarists know.
Hillstream loaches as a group have a real following in the planted-tank and biotope hobby, where their algae-grazing habit and need for strong current and high oxygen make them a specialist choice for high-flow setups, and Erromyzon species such as Erromyzon compactus and E. sinensis, both sold under "hillstream loach" names, occasionally reach the trade. Conservation data are limited — none of the five species in the atlas has enough data for an IUCN assessment, so the group's wild status is essentially unknown rather than settled.