You would be better able to determine/eliminate a cause for the pH cycle, if you tested at about 3pm and 3am. If the pH issue is caused by organism activity/photosynthesis, the pH should peak in the early/mid afternoon. Conversely, it should be at its lowest, late, late at night/before sunrise.
Just curious, are there any sources of lime in your pond? Cement walls? Crushed cement? Bricks? Mortar?
