pH can also vary throughout the day depending on the photosynthetic activity (consumes CO2...raises pH) or respiration (produces CO2...decreases pH). If you monitor pH continously you will typically notice pH to be lowest right when the lights come on in the morning because during the night photosynthesis is typically 0 and CO2 builds up overnight due to fish respiration, etc. This is why many reefers will run a reverse photoperiod on their refugium in an attempt to balance the CO2 levels between day and night.

What method are you using to measure pH? Are you using a probe or strips? How often are you calibrating?