Hillstream Loach (Sewellia lineolata)

Sewellia lineolata is a gastromyzontid hillstream loach native to the fast-flowing hill streams and torrents of central Vietnam and Laos, primarily found in the Mekong drainage. Its common names reflect both its pattern ('reticulated,' 'tiger') and its specialized habitat ('hillstream').

The body is dramatically flattened — an adaptation to life in powerful current — with enlarged, horizontal pectoral and pelvic fins that form a broad base, allowing the fish to cling to smooth rocks and glass against strong water flow. The underside is modified into a weak suction disc formed by lip modifications and fin positioning. The dorsal surface displays an intricate pattern of interconnected golden-yellow lines and spots against a dark brown-black background, creating a striking net-like reticulation.

In their natural environment, these loaches inhabit clear, fast-flowing, oxygen-saturated water over bare rock and gravel substrates with high light levels supporting dense biofilm (periphyton) and algae growth — their primary food source. The water is cool (typically 18–24°C) and very clean with minimal dissolved organics.

In the aquarium, Hillstream Loaches are specialized but rewarding fish. They require conditions that closely replicate their torrent habitat: strong water movement, high oxygen, very clean water, and hard surfaces for grazing. Standard tropical community conditions (warm, slow-moving) are inadequate and cause rapid health decline.

Hillstream Loaches have very specific requirements. Maintain pH 7.0–8.0, hardness 5–15 dGH, and temperatures 18–24°C. High oxygen levels and very strong water current are non-negotiable. Use multiple powerheads, a strong filter return, and spray bars to create the torrential flow they require.

A minimum 80-liter aquarium with smooth rocks, pebbles, and slate pieces provides essential grazing surfaces and resting spots. Strong, broad-spectrum lighting encourages algae and biofilm growth on rock surfaces — their primary nutrition source. Avoid soft substrate; they need hard surfaces to adhere to.

Feed primarily through allowing natural algae and biofilm to develop on rocks and tank glass. Supplement with algae wafers, blanched vegetables (zucchini, spinach, cucumber), sinking spirulina tablets, and occasional small live or frozen foods (baby brine shrimp, daphnia) for protein. They graze continuously.

Hillstream Loaches are peaceful but may be territorial with conspecifics in small tanks. In larger tanks (150+ liters) with multiple rock territories, several individuals can coexist. They are compatible with small, cool-water community fish that tolerate strong current: danios, White Cloud Mountain Minnows, small barbs, and cold-water corydoras.

Water changes of 30–40% weekly with temperature-matched water maintain the pristine conditions this clean-water specialist requires. Avoid adding them to tanks with poor water quality, high nitrates, or inadequate oxygenation.