Length beats litres
Ask the question as 'how long' instead of 'how big' and the answer makes sense: a hillstream loach needs a tank around 80 centimetres or more in length - the class of long aquarium sold at roughly 30 gallons - because everything this fish does happens along the bottom and across surfaces. It grazes over rocks, claims a favourite stone, and shuffles sideways to face down a rival, all in two dimensions. A tall cube holding the same volume gives it a fraction of the floor and stacks its world uselessly upward. When you are comparing tanks, put the tape measure on the footprint and let the litres look after themselves.
The flow is the real requirement
Tank size matters here mostly because of the water the tank must hold: cool, saturated with oxygen and moving fast, like the rocky streams these loaches are built for - their flattened bodies and sucker mouths are hydrofoils for clinging in current. A longer tank lets a powerhead or a river-tank manifold push a genuine one-way flow down its length, with calmer eddies behind the rocks where the fish can rest. In a short tank the same pump just makes a washing machine. Aim for water in the high sixties to mid seventies Fahrenheit, since warm water holds less of the oxygen this fish demands.
Rocks, algae and grazing room
What fills the footprint decides whether the size works. Smooth, rounded boulders - nothing sharp-edged - give each loach a grazing station and a sightline break from the others, and they should be in place and filmed with algae and biofilm before the fish arrive, because that film is a large part of the diet. Keep some surfaces in bright light specifically to grow it. A bare, sterile long tank starves a hillstream loach just as surely as a small one; a slightly smaller tank with mature, algae-dusted rockwork can honestly serve it better, though the long footprint with mature rock is the real target.
A group, in the right footprint
Hillstream loaches are social in their own sparring way and do best in a group of three or more, which is the other reason the footprint has to stretch: each fish holds a loose territory on the rocks, and too little floor turns their displays into constant eviction. Space lets the group spread into a pecking order and settle. Stock the rest of the tank for the same cool, moving water - the fish that thrive in a still, warm setup will be as out of place as the loaches would be in theirs - and fit a tight lid, because a fish built to climb wet rock in a current can climb glass and leave by any gap it finds.
Agree the build before it is built
A river tank is the kind of setup that gets designed on the back of an envelope and then argued about rock by rock - where the footprint goes, how the flow runs, whose favourite tall tank has to be passed over. It goes better as a shared plan with the dimensions, the rock layout and the stocking list written down where the whole household can see them. Putting that plan on the tank's profile through Family Sharing in KinsLe means everyone is literally looking at the same blueprint, and the loaches arrive to a finished river instead of a work in progress.