Is the water level in your pond getting lower, and you think it could be due to a leak in the liner? The first step you'll need to take is finding out if you actually have a leak, and then fix the problem. Here is how you can do it on your own.
Determining If You Have A Leak
The problem with determining if you actually have a leak in the pond liner is that the water level may be decreasing due to natural evaporation. The best way to find out if this is the case is to do a test.
Start by putting out a container of water right next to your pond, and marking where the water level is in the container. You will also need to mark the water level in your pond as well. Let the water sit in the pond without any pumps running for 1-2 days, and then verify the water levels of both the container and your pond. If they decreased by the same amount, you are losing water due to evaporation. If the water level in the pond decreased more than the container, you have a pond liner leak.
Locating The Leak
Allow the water levels to decrease until it slows down, which means that the water levels have lowered to the point in the liner where the leak is located. Check around the pond at water level to locate any small hole or rip in the liner that water can escape from.
If you are having trouble finding the leak, a trick for finding it involves using milk. Use a spray bottle to spray milk around the edges where the water meets the liner. If there is no leak, the water should remain cloudy. If there is a leak, you will be able to see the milk drift toward the hole. Mark the hole so you don't lose track of it.
Fixing The Leak
You can purchase a rubber patch kit from your local home improvement store that will have all the tools you need to repair it. You can actually repair a hole that is underwater, so there is no need to drain your pond even further. You simply apply the adhesive to the area where the hole is, and stick the patch on top of it. It doesn't use a traditional glue to hold the patch in place, but forms a chemical bond when the adhesive touches the rubber patch.
When your pond liner has many holes and it is beyond repair, you'll need to replace the liner completely. Reach out to a local company that specializes in pond liner manufacturing for help.