Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
pH is not a stand alone value. Many things in the tank can control the pH.
Test and post back with GH, KH of tap and tank.
Also, run some tap water into a glass and let it sit out 24-48 hours and test the pH in that.
If you think it is something in the tank (perhaps the gravel) then put a handful of gravel in a different glass and test GH, KH and pH every few days for a week (less if it changes sooner). Do this with each material from the tank that you think might have an affect on the pH, GH or KH.