These fish come from shade
In the wild, harlequin rasboras live in slow, tea-stained forest streams where a canopy overhead and tannin-darkened water filter the daylight before it reaches them. Their aquarium lighting should start from that picture: soft, warm-leaning, and broken up by planting rather than poured evenly over open water. A moderate full-spectrum fixture is the right tool, but it earns its keep at reduced brightness or filtered through growth above the fish.
The reward for getting this right is colour. A settled harlequin shows a rich copper-orange body with a crisp black triangular wedge, and it shows that colour out in the open, schooling in the middle of the tank. The same fish under stark, bright light turns pale pink-grey and spends its day in whatever shadow the decor provides — the species' first response to feeling exposed.
Diffusing the light you have
You rarely need a different fixture; you need a softer delivery of the one you own. Floating plants are the classic rasbora solution, spreading a living canopy that dapples the light below and gives the school a ceiling to feel secure under. Tall background planting at the ends of the tank creates shaded margins the fish can drift into. If the fixture has a brightness control, running it at a gentler setting for a slightly longer day suits harlequins better than full power for fewer hours.
A dark substrate completes the effect. Over pale gravel, reflected light bounces upward and the fish stay muted; over dark sand or soil-toned gravel, the school reads as glowing embers and behaves more boldly, because the floor no longer looks like open, exposed ground to them.
Hours and rhythm
Eight to ten hours of this softened light, on a timer, gives plants enough day and the school a predictable rhythm. Harlequins are most active and feed most confidently in the first part of the lit period, so overlapping the schedule with your morning feed makes observation easy. A gradual start helps: these are small, easily startled fish, and a school that bolts at switch-on day after day is a school living under low-grade stress. A simple ramp, or ambient room light reaching the tank before the fixture fires, smooths the transition.
Avoid leaving subdued blue night lighting on for long stretches as a substitute for darkness. The school needs a genuinely dark rest period, and perpetual twilight blurs the day-night pattern their behaviour is organised around.
Planting that shares the light
The low, soft regime harlequins prefer suits shade-tolerant planting — the slow, broad-leaved and fern-like species that grow attached to wood and stone, and the rosette plants that thrive in dimmer corners. These grow slowly under rasbora lighting, which is fine: the tank's look comes from the fish and the layout, not from rapid plant growth. Fast, light-hungry stem plants are the ones that struggle here, stretching and shedding lower leaves, so choose planting for the light the fish want rather than raising light for plants the fish dislike.
Algae is rarely a battle in a correctly dim rasbora tank; if it appears, look first at a timer that has crept longer or a floating canopy that has thinned, quietly letting full light through again.
Reading the school
A harlequin school is its own light meter. Tight, fluid schooling in open water with wedges black and bodies copper means the light is right. Fish scattered, pale, and parked individually in cover means something — usually brightness — is wrong. Adjust in that order and give the school a few days to respond before changing anything else.
When several people tend a community tank, writing down the dimmer setting and the floating-plant routine prevents a helpful relative from helpfully turning everything up, and KinsLe Family Sharing keeps that shared profile and care information with the fish so everyone works from the same details.