The short answer
Fish jump for a mix of instinct and discomfort: being startled, stress, poor water quality, or simply a species that naturally leaps. Whatever the trigger, the reliable fix is the same — fit a proper lid or cover — while you also test your water and reduce the stress that makes jumping more likely.
Fit a lid first
The most important step is physical: a well-fitting lid or cover stops a startled fish from ending up on the floor. Even a calm tank benefits, because jumping is often a sudden, one-off reaction rather than constant behaviour. Check for gaps around the filter, heater cables and feeding hatch, since fish find surprisingly small openings.
Reduce the reasons to jump
Fish are far more likely to leap when they’re stressed or the water is uncomfortable. Test with a liquid test kit — ammonia or nitrite irritates fish and drives escape behaviour — and keep up with water changes. Sudden movements, tapping on the glass, bright lights coming on in a dark room, and aggressive tankmates can all startle a fish into jumping, so keep the tank in a calm spot and avoid abrupt disturbances.
Species and settling in
Some fish are natural jumpers, and newly added fish jump more while they’re unsettled. Careful acclimation, plenty of surface cover such as floating plants, and a secure lid together make a big difference. If jumping continues in a covered tank with clean water, look again at stress sources — tankmate aggression is a common one, covered in why is my fish chasing other fish?. For persistent, unexplained stress, ask an experienced fishkeeping community for a second opinion.