Small fish, small exits
White cloud mountain minnows live in the upper water, swim in quick, darting bursts, and startle as a school — one fish bolts and the rest follow the same line, which may be over the rim. Males displaying and sparring add upward dashes of their own. What sets this species apart is scale: a white cloud is slim enough to pass through openings that are irrelevant to most community fish, including stretched net covers, wide mesh and the crescent gaps beside rounded filter pipework. Owners often have a lid and still lose minnows, because the lid was never the problem — the holes in it were.
Fine mesh and a flush fit
Cover the tank fully, then go around every cut-out — filter inlet and outlet, heater cable, airline — and close it with fine, secured mesh or precisely cut acrylic. Check mesh by eye against the fish: if a gap in the weave looks anywhere near the width of a minnow's body, it is too coarse. Press infill pieces down and confirm no corner lifts, since a light piece can be nudged by repeated hits from below. Lowering the water a few inches below the rim adds the usual margin and matters more here because white clouds jump from close to the surface, where they spend their day.
Keep the top covered during water changes as far as the siphon allows, and count the school afterwards for a few days after any maintenance that rearranged equipment; a changed filter position can open a new exit beside the old one's sealed spot.
Calm zones in a cool, flowing tank
White clouds come from cool, moving water and enjoy some current, but a small tank where the filter output sweeps the whole surface leaves the school permanently swimming against it, tiring and flightier. Angle the outflow or baffle it slightly, and plant one end of the tank so the fish have a slack-water refuge to retreat into rather than scattering upward when startled. A settled, adequately sized school also helps: a handful of minnows in open water is a nervous group, while a larger school over plants behaves like a single calm organism.
Their cool-water preference cuts the other way in summer: a tank that warms well beyond their comfort makes white clouds restless, and increased jumping in a heat spell should prompt a temperature check and gentle cooling of the room or tank rather than a bigger lid alone. Sudden light is the other reliable trigger — approach and light the tank gradually where you can.
A fall, and what it is telling you
A minnow found out of water goes straight back in with wet hands and low light; small fish dry fast, so speed matters, and any fish that then struggles to swim or is found repeatedly near one corner gap has shown you the exit to seal. Repeated losses from a covered tank mean a hole remains — inspect at water level with the lights on, looking for light lines around pipework.
Restlessness that is new, in a covered and sealed tank, still earns a water test and a filter check before it is dismissed. To keep the fiddly mesh work from depending on memory, KinsLe Care Teams lets you schedule the white clouds' lid-and-mesh inspection after every clean, with the completed check marked by a timestamp.