Sewellia is a genus of hillstream loaches native to fast, clear, well-oxygenated rivers and streams in Vietnam, Laos, and Cambodia, mainly within Mekong and central Vietnamese coastal drainages. Their bodies are flattened and disc-like, with enlarged, fan-shaped paired fins that fuse into a suction-cup grip letting them cling to smooth rock faces in strong current; they spend their lives grazing algae and biofilm off submerged stone rather than swimming freely in open water. Species in the atlas range from about 1.5 in up to 3 in.
The genus is a favorite among high-flow and aquascape keepers for its bold mottled and banded patterns, seen in species like the butterfly loach, elongate hillstream loach, reticulated hillstream loach, marbled hillstream loach, winged hillstream loach, and porcupine hillstream loach. They are active, sociable fish that graze constantly across glass and hardscape, but they need cool, highly oxygenated, current-swept water delivered by powerheads or a strong filter return rather than the calmer conditions typical of many tropical community tanks. Conservation data lean toward Not Evaluated, but the tally also includes 2 Critically Endangered and 1 Endangered species, underscoring how vulnerable these torrent-dependent habitats can be.