A fish that only makes sense in a group
Harlequin rasboras are small, copper-orange fish with a bold black wedge on the rear half of the body, and everything about keeping them starts with the shoal. Six is the bare minimum ever quoted; eight to ten or more is where the behaviour you bought them for appears - the group holding station together in the current, males sparring gently and flashing their colour at each other. A first tank of 20 gallons or more gives a proper shoal room to move as one body. Lone harlequins and pairs fade, hide and fail, so buy the group complete, from one shop tank, once the aquarium is fully cycled.
Soft, warm, slow water
Wild harlequins live in shaded, slow-moving forest streams in Southeast Asia, and the tank should rhyme with that: warm water around 73 to 77 degrees, soft and slightly acidic where you can manage it - though tank-bred harlequins accept ordinary neutral water as long as it is stable - and gentle filtration that does not pin the shoal in a corner. They colour up dramatically better in soft water over dark substrate, which is why shop fish often look washed out and then transform a fortnight after coming home. Driftwood and leaf litter suit the setup both visually and chemically, releasing the gentle tannins of their home streams.
Plant the edges, leave the stage open
Harlequins spawn on the undersides of broad leaves and shelter in planting, but they display in open water, so arrange the tank accordingly: dense stems, crypts and anubias along the back and sides, a canopy of floating plants to dim the light, and the front centre left open as the shoal's stage. Floating cover is not cosmetic for this species - bright, exposed tanks produce pale, nervous harlequins that hug the plants. Fine gravel or sand in a dark tone completes the effect, and the fish will repay the design by spending their days visible, mid-water, in formation.
Peaceful community citizens
Few fish are easier to house socially. Harlequins ignore bottom-dwellers, do not nip fins, and are too quick and too mild to trouble anything - their only requirement is that nothing in the tank can eat them or outcompete them brutally at feeding time. Good neighbours are other small rasboras and tetras, corydoras, kuhli loaches, small gouramis and peaceful shrimp colonies, with the usual caveat that newborn shrimplets may be hunted. Avoid boisterous barbs and large cichlids. Feed a small omnivore flake or micro-pellet, crushed to fit small mouths, with daphnia or baby brine shrimp as regular extras, and the shoal will feed as a group at the surface in one of the prettiest sights in small tanks.
Settling in and the months after
New harlequins often arrive pale and shy; give them dim light, the floating cover in place before they arrive, and a day without food, and colour usually returns within the week. Males are slimmer and more vividly marked than the rounder females, and a settled group in soft water may begin spawning behaviour over broad leaves, though raising the fry is a separate project. Long term these are sturdy fish that can live five years or more, with losses usually traceable to an uncycled start or to being kept in a group too small to feel safe. A tight corner cluster in a new shoal can mean simple nerves, or a water problem worth testing before anything else. For that distinction, put a follow-up question to KinsLe's Kimmy; she answers using the KinsLe Answers library.