One fish, times twelve
Everything about a harlequin rasbora is collective. The copper body and black wedge are a uniform; the fish school mid-water in a tight, polished formation, and when something startles them they do not scatter as individuals — the whole school fires off in one direction like a single animal making a single decision. On fireworks night that unity cuts both ways. A confident school absorbs a flash, flinches, and settles in seconds, each fish reassured by eleven calm neighbors. A school that does break does so at speed, and its first obstacle matters: glass, a filter intake, a bare corner. Harlequins are also mid-water fish with no interest in caves or the substrate, so the cover that helps them is vertical and leafy — planting tall enough to hold the school inside it. Your plan follows from those three facts: keep the group whole, buffer its dash lines with plants, and flatten the flashes that trigger the dash.
Preparing the school's water in the afternoon
Start by counting, discreetly, at feeding time — the plan assumes the full school, and if numbers have quietly thinned over recent months, tonight is not the night to fix it, but the knowledge shapes how much buffering you add. Look at the tank from the school's lanes: mid-water, the length of the tank. Where does a darting school land at each end? Ideally the answer is a wall of stem plants or a thicket of wood and leaves at both ends and along the back, so the first thing met is foliage that gives. Add a temporary bunch of stems or a spare plant if one end is bare glass. Fit or confirm the back background, which kills the see-through effect that makes flashes feel like they come from inside the tank. Set the room for a low lamp left on from dusk, feed the school its normal meal early, and from then on the tank's day is over — no water change, no net, no rearrangement with the display approaching.
During the evening, from a chair, not the glass
Harlequin watching during fireworks is done from across the room. The school will flinch at the bigger bangs; what you are looking for is the recovery, the formation loosening back into its easy hover within a minute or so. A school that stays balled tight and low for the whole evening is enduring rather than enjoying, but enduring is acceptable for one night, and the correct response is still nothing — no tapping, no light changes, no well-meant inspection with a torch. If you are going out to watch the display yourself, the kindest arrangement is a household member who stays in, keeps the room's lamp on, and glances at the tank twice in the evening rather than hovering over it. Keep cats and excited dogs out of the room; a paw at the glass after a bang teaches the school the flashes come with predators. Leave the lid on, the filter running, and the plants exactly where they are.
After: formation, fins, and breakfast
The morning verdict comes at feeding. A harlequin school that rose through its night well pours out of the plants in formation and feeds mid-water as one; check as they turn that flanks are unmarked and the copper color is back at full strength. A school that feeds in fragments, with one or two fish holding back in the stems, deserves a dim, quiet day and a small water change, and usually re-forms by evening. Note anything you saw — which bangs moved them, whether the new planting at the bare end earned its place — because this school will meet fireworks again and its record should improve each time. Dividing the work keeps it reliable: with KinsLe Care Teams, the afternoon buffer check, the two evening glances, and the morning formation watch can sit with different people as timestamped duties, so the school gets its quiet, covered night even when the household is celebrating.