A schooling fish first, a tank-size question second
Harlequin rasboras are genuine schooling fish: they hold together, turn together and feed together, and a fish kept in a pair or trio is a shadow of the same species in a proper school. Plan on at least eight to ten fish, and size the tank for the school rather than the individual. An adult harlequin reaches only about two inches, but ten of them moving as one body need a tank with real horizontal length, which is why ten gallons is the working minimum and a twenty-gallon long is the size where the school truly settles, spreads out and starts behaving like the wild shoals it descends from.
What the minimum tank needs to provide
Harlequins come from slow, shaded, soft-water streams, and the tank should echo that: a longer footprint over a tall one, gentle filtration that does not push them around, and planted edges with open water in the middle where the school can hold station. Dimmer light, or floating cover that breaks it up, brings out their copper-orange colour; under harsh bare light the same fish look washed out and stay nervous near the bottom. They spawn on the undersides of broad leaves, so a plant or two with substantial leaves gives even a non-breeding group the furniture its instincts expect, and you may be rewarded with the upside-down egg-laying dance even if no fry survive in a community setup. Stability matters more than chasing exact stream chemistry: a school settled into steady conditions it has lived in for weeks will out-colour one whose water is adjusted again and again in pursuit of a number on a chart.
Stocking around the school
At the minimum size, the school is the tank — resist adding a second mid-water species competing for the same swimming lane, because two half-groups of different fish look restless where one whole school looks serene. A small group of bottom dwellers that stays out of the rasboras' level works far better in a ten-gallon, and in a twenty the school itself can grow to a dozen or more, which sharpens the schooling behaviour that made you choose them in the first place. Numbers first, variety later: a thin school of harlequins scattered among several other species pleases nobody, least of all the rasboras, whose confidence visibly drains away when their own kind are few. Cherry shrimp and small snails make safe companions at this size precisely because they work the glass, plants and floor, leaving the school's mid-water lane entirely to the school.
Why a stable group needs a shared list
A school is fed, counted and checked as a group, and its numbers drift in small steps — one fish lost, three added after a trip to the shop — in ways a busy household easily misremembers. Because a school is fed, counted and checked as a group, it helps when everyone in the house works from the same stocking list rather than guessing how many rasboras there should be. With the school's numbers, tank size and care notes shared through KinsLe Family Sharing, whoever feeds them can spot a missing or listless fish early, because the whole family knows exactly what a complete, healthy school looks like.