I think you are taking a large risk, if you let it sit empty over the winter.

You might want to wait till the water cools -- but is not unendurably cold -- and let it leak down far enough to tell where the leak maybe. If it leaks below all fittings, then you know it's in the liner. If you do this in Sept, you can still wade around, and find and patch the leak.

You can maintain the water with plain 6% household bleach.