pH and alkalinity can definitely change due to many conditions. Acid rain and the environmnet around your pond will impact the pH levels for sure. Adding aglime will increase the alkalinity and help buffer the pH. The challenge is that as your water source is refreshed through rain and runoff your pond is going to tend toward levels it was at before adding the aglime. We put 4 tons of aglime in our 1 1/2 acre pond last summer and it really balanced the pH nicely. Then we got a very rainy period and replaced hugh volumes of the water. I'm guessing we effectively replaced the whole ponds water supply several times over in less than 2 months time. Needless to say the pH went out of wack again even though our alkalinity levels stayed good. Before the winter freeze we were still acidic but somewhat improved. It takes some time to buffer the pH and I think we may be adding more aglime again this spring. Will have to wait and see as the spring melt again causes major refresh of our water.