theyt raise it or lower it to whatever the number is
example- my tank is 7.0 if i put in a powder that sets it to 8.0 id raise to that
but i want a high ph tank so akaline water would be there
I would look for peat in your LFS. It will make the water acidic and soft as well as adding a yellowish tinge to the water.
Take it from one who learned his lesson the hard way quite a few years ago . . . IMO fooling with chemicals to adjust the pH is a sure way to get into trouble with pH swings.