Celestial pearl danios are among the smallest fish in the hobby, and their jumping problem is a problem of scale. A gap that looks sealed to you, the crescent beside an airline or the slot where a filter bracket meets the rim, is an open gate to a fish barely longer than a thumbnail. These are also shy fish from densely vegetated water, and most of their jumping is fleeing: a male chased by a rival, a female escaping persistent courtship, or any individual startled in a tank too bare to hide in. Your job is to shrink the exits to nothing and give frightened fish a sideways escape into plants.
Chases, not wanderlust, drive most jumps
In a settled group, celestial pearl danios spend their time displaying, sparring and grazing among plants in the lower half of the tank. The jumps happen at the peaks of that social life. Males posture and chase, especially in morning light, and a fish losing the argument darts for the nearest refuge; if the plants are thin, the dart continues upward and out. Keeping more females than males, and breaking the tank into planted territories so a chased fish can vanish from its pursuer's sight within inches, ends most chases at water level. A single pair in a bare nano tank is the worst arrangement: nowhere to hide, constant pursuit, and a rim within easy reach.
Mesh the rim at this fish's scale
Fit a lid, then go around it with this species' size in mind. Cover filter and heater openings with fine mesh or foam trimmed to sit snugly, check the corners of sliding glass covers where a small overlap gap often hides, and close the feeding hole habitually rather than leaving it open after use. Lower the water level slightly below the rim so a vertical dart loses height before it reaches glass.
Plant like they mean it: thick and low
Dense planting is jump prevention for celestial pearl danios, not decoration. Mosses, fine-leaved stems and low thickets give every fish a hiding route that starts the instant it is chased, and a fish that reaches cover stops fleeing. Floating plants soften the light from above, which calms a species that startles at overhead movement, and they reduce the surface activity that leads to overshooting darts. Keep flow gentle, since fish fighting a strong current tire and behave erratically, and position the outlet so there is a calm zone at the surface. Add tankmates, if any, with restraint: anything large or fast turns a shy community permanently defensive.
Stable, quiet routine
These danios react badly to upheaval. Make water changes small, regular and temperature-matched, test promptly if the group suddenly goes skittish, and approach the tank slowly, since a face or hand looming over a nano tank fills the fish's whole sky. Feed small foods at the surface calmly and let the group come up for them rather than tapping the glass to summon them. If one fish jumps repeatedly while others stay calm, suspect relentless chasing of that individual and rearrange planting or rebalance the group rather than only improving the lid.
If one gets out
A fish this small dries out frighteningly fast, so return it immediately on a wet fingertip or soft spoon, straight into the tank, and watch for it rejoining the group among the plants. Search the rim and cover glass too, since a danio can land on a warm lid and be missed. Then find the exit, which at this scale is usually a cable gap. For the broader escape safety net every pet owner hopes never to use, KinsLe's Lost Pet Alert System sends a free alert to nearby Circle of Love members when a pet goes missing.