Don't add PH buffer, for love of the hobby!!!
PH buffers are worthless, most people don't even check thier PH.
All those PH buffers are is an alkalinity solution. While it will increase your PH, it will do so only over a small amount of time causing you to keep adding it. What will result is a sky high alkalinity(things start to die if it gets too high) and still low PH. Don't chase numbers, anywhere between 7.6 and 8.5 is fine for PH.
Typically low PH just means you need more O2 added to the tank(build up of CO2). This can be accomplished in many ways, from adding an outside airline to your skimmer, more surface circulation, to adding a fan above your tank, or just plain opening a window.