wow...2 years ago i made the same mistake...too engross about pH. in the end realise the pH meter not calibrated.
so make sure your meter register 7 in buffer 7 or if you use a pH kit, that its realiable one and not expired.
measure your tank pH during photoperiod i.e a few hour after main light's on.
but if you ask me...i rather spend my time stabilising these 3 parameters KH,calcium and salinity.