pH is not a stand alone value. The various minerals in the tank and other things control the pH.
For you to control the pH, the first step is to control the minerals in the tank.
1) What kind of fish are you keeping? What is their preferred GH range?
2) What is the GH and KH of the tap water?
Get a gallon of distilled or reverse osmosis water.
Make some tests blending RO + tap. See what the GH, KH, pH (and tds if you have a meter) do with the different blends.
try 25% RO + 75% tap
50/50
75% RO + 25% tap.
If you are keeping black water fish, then add peat moss to the recipe that brings the GH and KH closest to what the fish want.
Once you have figured out the recipe there are 2 things to do.
The first is to switch the tank over to that kind of water. Do not do this all in one day. Do small water changes more frequently so the change is gradual. It may take a month, but that is the safe way to do this.
Example: first week do 2 water changes of 10%.
second week do 2 water changes of 25%.
and so on.
After the tank is running with the mineral levels the fish want, you will always have to prep the water ahead of time, using the recipe you worked out.
I would run the RO and tap water (+ dechlor) into a garbage can (Rubbermaid Brute, on wheels) and add a knee-hi stocking of peat moss. Add a fountain pump to circulate the water overnight, and an aquarium heater to warm the water. Next day, do the water change.