This won't apply to a great degree as of yet since you don't have much algae growth...
Photosynthesis during the day absorbs the CO2
in the water and hence causes the pH to increase. So at the end of day, around when lights go off, the pH in the tank will be at its highest.
Then at night there is respiration which releases CO2
into the water, so you get the opposite effect of pH going down. This will continue until lights come on again, so around that time is when pH will be at its lowest.