Leaking liner?

Finding the leak:

Chances are your liner is not leaking at all if you are losing water. Evaporation is usually the cause of slowly lowering water levels. If the water level falls significantly overnight, you are most probably losing it because water is falling off the back of a waterfall or if you have a fountain, the wind has blown the water out of the pond. Or if the water is falling from too far up, it is splashing out. Check all of these things before you suspect a leak in your flexible liner.

If you have finally determined that your liner is leaking, your biggest problem is finding the leak.
Most liners have some sort of patching material, much like patching a tire, but until you find the leak, you cannot patch it. I have found that pumping the water out of the pond and then running water under the liner is the best way to find that ‘unfindable’ leak. Here in New Orleans, the water will come up through the hole when I get water under the liner. I know it is a pain in the neck, but I have yet to find a better way.

At least when you have found the leak, the patching of it is a piece of cake.