Harlequin rasboras come from slow, shaded forest streams, and a tank decorated for them should feel like one: subdued light, plants with real leaf canopy, wood overhead in places, and a school that can travel the length of the aquarium as a single copper-and-black ribbon. Decor choices either support that group movement or chop the tank into fragments the fish never settle in.
Broad leaves, including one job only they can do
Cryptocoryne species are the centrepiece plant for harlequins, not just for looks. When harlequins spawn, the female turns upside down beneath a broad leaf and the pair deposit eggs on its underside, so a mature crypt with wide, firm leaves is functional furniture. Plant several in a loose group mid-tank, rooted in the substrate, and give them time to fill in before judging the layout.
Behind the crypts, fine-leaved stem plants build a soft green wall that makes the school feel protected from behind. Anubias on wood adds a second tier of broad leaves at a different height, and its slow growth keeps the arrangement stable between prunings. Avoid carpeting the whole floor; harlequins live in the middle layer and need the bottom left comparatively plain.
Wood, litter, and subdued corners
Slim driftwood branches angled through the upper half suggest the fallen timber of their home streams and give the school reference points to circle. A few smooth stones and a modest scattering of dried leaves in one corner deepen the stream-floor feel and release gentle tannins that many keepers find bring out the fish's colour. Replace leaves as they soften and break down, adding them to the same corner so the rest of the floor stays easy to vacuum.
Light dimmed from the surface down
Harlequins in bright, bare light look washed out and hover nervously; under a partial floating canopy they colour up and school calmly. Float water lettuce or similar rosettes over part of the surface, or let tall stem plants lean across the top, keeping a gap for feeding and gas exchange. The goal is dappled shade, not darkness, so lower plants still grow.
The open lane the school travels
A school needs planting it can disappear behind
Harlequins judge safety by what stands between them and open glass, so the planting at the ends matters as much as the broad leaves in the middle. A dense end thicket gives the school somewhere to pour into when a hand, a net, or a sudden shadow spooks them, and a group that knows it has a bolt-hole spends the rest of its time calmly in view. Cryptocoryne also rewards patience: newly planted specimens often shed leaves while their roots settle, then regrow from the crown, so do not rip out a bare-looking crypt in its first weeks. Mark its position, keep the lane clear around it, and let the school use the wood and stems until the new leaves come through at the size the spawning fish need.
Reserve an unobstructed midwater corridor along the full length of the tank. That lane is where the group behaviour you bought harlequins for actually happens: the school pours down it, wheels at the end, and pours back. Feed into the lane's edge so the whole group rises together, and resist filling the middle with tall ornaments or a feature rock. Writing down the floating-plant thinning schedule in KinsLe Family Sharing keeps the light soft and even, whoever in the family last tended the tank.