Seems a bit odd to me. There is a chance that your water company adds Co2 to you tap water. Co2 will lower the pH of your water. It will eventually out gas from the water resulting in you pH rising.
Do a test. Put some tap water (or whatever your water source may be) in a glass. Immediately test the pH. Let the glass of water sit overnight. Test it again in the morning. Compare the results. They should tell you if there is Co2 in your source or not.
The goal is to maintain a stable pH. Your fish will adapt to just about any level providing it stays constant. I would suggest you try not to alter it. It may drop a bit after a water change, but it will quickly rise back up to it's sans Co2 level as the Co2 out gases.
Co2, or lack thereof, is altering your pH.