Start with the school itself
A harlequin rasbora alone or in a trio is a nervous fish that hugs cover and barely shows its colour. The same species in a group of ten or more becomes a different animal: confident, copper-bright, travelling the tank in loose formation and breaking into sudden synchronised darts when food appears. If your rasboras are inactive, count them before you change anything else — no ornament compensates for a school that is too small to feel safe.
Dark below, soft light above
Harlequins come from shaded, peaty forest streams, and their colouring answers to that world. A dark substrate and dark background make the orange-copper body and black wedge glow instead of washing out, and fish that feel visually secure swim openly. Floating plants or tall stems soften direct light into the dapple they evolved under. Some keepers stain the water lightly with botanicals such as dried leaves, which suits this species well and adds a slow release of grazing film on the leaf surfaces.
A planted border around an open lane
Arrange plants along the back and sides and keep the centre open. The school will use the lane like a road, running its length in formation, then peel into the plants when startled and regroup — exactly the rhythm to aim for. Broad-leafed plants deserve a place in the border: harlequins spawn by gluing eggs under a leaf, and even outside breeding condition, pairs inspect and posture around broad leaves in a way that adds constant small activity. A gentle current along the lane, from a softened filter outflow, gives the school something to lean into without fighting the flow.
Small food, scattered wide
Fine foods suit their small mouths. Scatter micro-pellets or crushed flake across a wide stretch of surface so the whole school rises and darts together rather than queuing at one corner, and offer small frozen foods such as daphnia or baby brine shrimp a few times a week for the same hunting response. Feed little and often where your schedule allows; two small scatter-feeds keep a school busier than one large one.
Reading a school that is not thriving
Washed-out colour, a fish hiding alone in the plants, or a school that startles at every shadow are stress signals, not temperament. Check the water first — harlequins prefer it warm, soft and clean — then look at tankmates: anything large, fast or nippy will pin a school down. Single out persistent bullying and relentless chasing rather than the normal brief squabbles over position. Keep the routine that supports all of this visible in KinsLe, where Care Teams can split the scatter-feeds, leaf replacements and filter care across the household, each job confirmed with a timestamp.