your pH will fluctuate during the lights on period vs your lights off period. the pH will increase during the day due to photosynthesis of organisms and micro organisms in your tank. then it lowers during the night because photosynthesis is not occurring, thus causing the pH to lower. what kind of pH results are you getting and what time of day are you taking the measurements? is it a consistent time or sometimes in the morning and sometimes at night? i would recommend to take a few samples throughout the day and see if this is what is occurring. if you have a refugium, setting the lighting periods opposite of one another should correct this problem. if not or its some other problem you can try to use something like Kent Marine "Superbuffer dkH". it is supposed to maintain a fairly constant pH of 8.0 to 8.3. goodluck.
