What did you add to the tank - aside from your regular water changes - that caused it to be so high?
What additives did you add?
What size is the tank?
The best answer I can give you, is you do NOT want to add a chemical to bring it down quickly!! Changing pH rapidly is a sure way to KILL your tank.
The BEST way is through water changes until you get it to where it should be.
Chemicals will lower it BUT their effect is almost always temporary and the pH will increase again - in short order- and taking your fish/livestock on a rollercoaster pH ride is extremely dangerous, so please do not use any chemicals..use only water changes to lower it!!!!
API is okay but not the best you can buy for a reef tank.
Are you using the High Range pH kit?? (you need to be)
Were you previously getting a lower pH reading? Is this a sudden increase for you?
What are your Calcium, Alkalinity & Magnesium readings?
I would stop with the marine buffer, you should not need it if you are changing your water on a regular basis.
I would suggest you change 10% per week, not per month, and you should start tonight or tomorrow to try and get that pH down through water changes.
You might want to do more than 1 water change this week to get that pH down!
No more buffer and a good idea to invest in some quality test kits for all of your parameters..Ca, Alk, Mg..Salifert brand are good kits as are Elos and LaMotte
probably no problem at all. PH spike could be a few things. Just do a 15 gallon water change tomorrow and then test the water the next day. don't worry. its usually as simple as that . good luck. and you don't have to go and test evrything, waste of time. if your using r/o water then the PH is perfect out of the gate. but using a buffer never hurts.