This can be very difficult! However, when I had the problem I allowed the water to leak away until it stabilised at one level, then with care I applied a scrubbing brush at water level to clean the liner and left it to dry. \n\nNext I cut two inch widths of liner (in my case from left over stuff from when I'd originally installed the liner) and using a glue provided by the pond shop, I stuck this on all the way round at water level, leaving no gaps and smearing glue along both edges of the fitted strips. It took a long time and a lot of patience, but it worked.\n\nFinal tip: Lie on planks laid across the pond as you do this work!