Tetras jump, and they aim for gaps
Tetras are surface-aware, startle-prone fish, and a sudden shadow, a squabble or a lights-out click can send one straight up. In a well-covered tank that jump ends against glass or acrylic and the fish drops back in, shaken and fine. Losses happen at the exceptions: the corner of a cover glass left sitting proud, the generous cut-out around a filter pipe, the feeding hatch not quite clicked shut. A tetra needs a slot barely wider than its own body, and it will find that slot in the dark on the one night it lines up. The weekly check exists to make sure there is no slot to find.
Walk the whole lid line, in order
Once a week, work around the tank in one direction and physically press every cover panel, condensation tray and hatch down into its seat. Water changes are the usual culprit — lids get lifted, slid and replaced at an angle, and a panel can look closed from the front while its back corner rides a centimetre high. Run a finger along the back edge and the corners, the places you cannot see from your usual chair, and check that condensation trays have not warped into a bow that lifts their middle. If a panel rocks, it is a gap wearing a lid's clothing.
Block the equipment cut-outs
The holes cut for pipes and cables are made for the bulkiest equipment and leave crescents of open water around anything slimmer. Go through them one by one: the gap beside the filter inlet and outlet, the notch the heater cable exits by, the airline's corner. Block each with what fits — a piece of coarse filter sponge wedged around the pipe, a square of craft mesh taped or trapped under the lid edge, a flap cut from a plastic lid. The test is simple: no opening anywhere on the surface should be wider than your slimmest tetra is thick.
Water level is part of the lid
A tank filled to the brim puts the surface within jumping range of every gap, including ones too high to matter at normal level. Keep the waterline an inch or so below the rim; that headroom robs a jump of its escape velocity and means most leapers fall back before they reach cover height. It also buys tolerance for the lid checks themselves — a slightly proud panel over a lowered surface is an inconvenience, while the same panel over a brimful tank is an open exit. Top up evaporation promptly so the level you planned is the level you have.
Connect jumping to what startles them
A week with repeated jumping attempts is feedback, not bad luck: something is startling the shoal or the water is troubling them, and losses concentrate in tanks where fish already want to leave. Note any morning you find a fish at the surface acting spooked, test the water if it repeats, and soften sudden light changes where you can. KinsLe Care Teams can put the lid walk, the cut-out check and the waterline mark on the tank's weekly duty list, so the five minutes that keep tetras in the tank survive changes of routine, helpers and seasons.