Hi,
Take a 5 gal pail... fill with tap water right from the source . can be after 10 stage water filter... measure your pH in the pail. Add an airstone and let aerate (age) 24 hrs.Measure again. When the pH starts low and goes high its usually from dissolved gases like CO2 being present. Aerating blows off the CO2 and its the same thing that happens in your tank by the filters. The pail experiment will tell you alot ..
hth,
al