Don't add buffer to stabilize pH... that's a no-no.
What a buffer is to increase pH in a saltwater tank is sodium carbonate. It will temporarily increase your pH but it will also add alkalinity to your system - which your tank probably needed. Don't add alkalinity to your system without knowing your calcium and pH. Make sure that your magnesium is around 1300-1350mg/L before you start dosing anything...
API kits are OK, but if you are going to test and regularly adjust your water parameters, you need something a little more accurate - like a titration style test kit such as those sold by Seachem. Titration style kits are much, much more accurate than a simple drop method...
Don't make the mistake that many aquarists do and invest thousands of dollars in equipment, only to use a cheap test kit.