You dont need to worry about the Ph level in your freshwater top up water.
If you want to raise it in the main tank, just add baking soda.
add 2 teaspoons per 300 litres and test after that.
YOU ARE TESTING FOR ALKALINITY, NOT PH.
iF YOU GET YOUR ALKALINITY TO 8 OR 10 DKH YOUR PH WILL BE PERFECT UNDER NORMAL CIRCUMSTANCES.
RUN YOUR SKIMMER ALSO AS AERATION RAISES PH TOO.