You need to inspect the pond liner for any damage, no matter how small, such as tears or punctures. If you need to stop a leak quickly, consider spraying the pond's edges with milk from a squirt bottle. The milk will muddy the color if there is no hole in the water. And if there is a hole, it will move toward it.
Slurry bentonite or granular bentonite applied to the surface of a leaking pond containing water will work to prevent water loss.
Pond cleaning is pond-specific. Decorative ponds may only need cleaning once a year. Larger ponds used for fishing or swimming must be cleaned every two to four weeks.