You are sure you face repairing your pond liner. Before you panic, make absolutely sure it is a leaking liner and not another problem that is causing your water level to drop. Check your waterfall, check your hoses, check your spitters. Turn your pump off, fill up your pond. Watch it overnight. Did the level drop? If so, you probably have a hole in your liner.

The first thing you must do is find that leak in the liner.

Finding the leak
Finding a liner leak can be one of the most frustrating jobs on Earth. The liner is black, usually dirty and wrinkled. The leak should be right at the water line, because water can't leak out below that. Or can it? Here in New Orleans, our water table is only inches below the ground, so often there can be a hole in the bottom of the liner, but ground water pushes the bottom of the liner, so a leak could well be on the bottom and ground water is seeping into the pond.

Pour some milk in the pond. If the water is leaking fast, the milk will make a trail to the leak. But if the water is leaking that fast, you have a huge tear and you can probably see it.

Pour some fine sand in the water. The sand will follow the water to the leak, but as above, if the tear is that big, you should need no help finding it.

Sometimes you cannot find the leak no matter how hard you search. I have resorted to this method as a last resort. Pump the pond water out. Remove all fish, plants, pots, lights, etc. Using a wet vac, vacuum the bottom to be sure all the fish waste, leaves, and other organic matter is gone. Rinse and wet vac again. Make sure the bottom is dry, even if you have to wait several hours or get the hair dryer out. When you are positive it is dry, stick a garden hose under the liner and turn on the water. Watch the liner carefully. Soon you should be able to see water coming up from underneath. You have found your leak. Dry it again and patch with patching material. Replace all pots, plants, fish, lights and refill the pond with water.