To offer a definitive answer, we'll need a lot more detail about your cycle (type, duration, current readings), water (source, pH, any other info available), what you've been adding etc.
Generally, I'd suggest a large PWC and let the pH take care of itself. Since you're concerned, take a pH reading of your tap water after it has sat out for several hours (to let any Cl2 gas off). If you're committed too using the buffer, you should probably already have a good idea of what it takes to match pH between replacement water & tank water. If a PWC won't bring the pH in line, add more buffer to tank, but do it SLOWLY (shoot for <1/2 pH unit per day). Once you're set, be careful about pH adjusting your replacement water for changes & replace evaporated water w/ RO (if you're still really concerned about pH).
Of course, this advise is all null and void if you're using CO2 injection to aid plant growth (another one of those pesky details that should be included).