If you don't notice any obvious leaking then I would say go for pumping water into it. You can find some real reasonable prices on pumps on E-bay I bought a brand new 4500 gph pump for my Koi pond that uses very little electricity. During the summer I pump water from my well as I can lose up to 28 inches during the 6-8 week dry spell we always seem to have. I am about 8 inches behind on rain for the year and the pond is down about 6 inches right now.

Bob in North Alabama