pH normally fluctuates between day and night. It will be lowest first thing in the morning when you turn the lights on, and highest at the end of the day when you turn the lights out. This has to do with algae photosynthesizing and removing carbon dioxide from the water. In the absence of light, photosynthesis slows down and carbon dioxide builds up. It bonds with water to create carbonic acid, which drops the pH. This is why when the tank is dark, pH drops. When the light comes on again, more photosynthesis starts up, the carbon dioxide is used up, and the pH rises.