Seachem's "Reef Builder" shouldn't have any impact on PH, only KH. I have been using Seachem's buffer products for several months now and understand that the reef builder was designed with higher PH tanks in mind. I would recommend testing PH prior to lights going out, then when lights go out add "Reefbuilder" wait about 1/2 hour and retest PH, check for any rise, if any it should be very minor.