The short answer for a hillstream loach
Plan on a 20-gallon long tank as the minimum for hillstream loaches, and choose a 30- or 40-gallon long if you can, because this fish lives its whole life pressed to surfaces. Its flattened body, wide fan-like fins, and sucker mouth are built for gripping rock in fast, oxygen-rich streams. A tall tank with a tiny floor gives it almost nowhere to graze, however impressive the gallon figure sounds. What counts is a broad base covered in smooth stone, steady current sweeping across it, and enough separate perches that several loaches can each claim a rock.
Why current and footprint decide the tank, not depth
In the wild, hillstream loaches shuffle across boulders in water so shallow and fast that larger fish cannot hold station. Their bodies let the current pin them down while they rasp algae and biofilm from the surface. Recreate that bargain at home: strong, well-oxygenated flow, a mature tank with film already growing on the stones, and cool, clean water. A newly set-up sterile tank is a poor home even at the right size, because there is simply nothing on the rocks for them to eat yet.
Groups behave better than singletons. Three to six loaches will spar briefly over favourite stones, chase, and then settle, and that harmless squabbling spreads across a wide floor. On a cramped base the same behaviour turns into one fish guarding the only good rock while the others cling to glass in the flow's edge.
Building the river layout inside the footprint
Lay rounded river pebbles and flat stones across most of the base, leaving small sandy gaps between them. Angle a powerhead or the filter outflow along the length of the tank so the current travels over the rocks rather than blasting one corner. Add a few pieces of wood and hardy plants in slower pockets for variety, but do not fill the floor with sharp gravel or jagged decor that can abrade their soft undersides. A tight lid helps because a startled loach can climb glass surprisingly high on its sucker mouth.
Lighting that encourages a thin film of algae on the stones is a feature here, not a flaw. Resist scrubbing every surface clean during maintenance, and rotate which rocks you wipe so some grazing is always available. Stable, mature water matters more than chasing spotless glass.
Signs this loach needs more room or flow
A hillstream loach that spends its days on the filter intake or at the waterline is often seeking stronger flow and oxygen, not exploring. Faded patterning, constant hiding, or one individual monopolising a single stone also point to too little usable floor. Tankmates should be peaceful fish that enjoy similar cool, moving water; slow warm-water species make poor partners because the conditions that suit one side stress the other.
Because this setup depends on powerhead direction, stone placement, and leaving algae in place, write those details down where helpers can see them, and KinsLe Family Sharing keeps that river layout recorded in the shared profile so a well-meaning clean does not undo it.