I think what is happening is that during the night, with no photosynthesis going on the water is absorbing CO2 from the air. Then during the day the plants use up CO2, leaving the water with less CO2, therefore with a higher PH. I would say it is going from around 4 ppm in the morning to around 2.5 ppm by the end of the day. During the day the water is still absorbing CO2 from the air, but the plants are using it faster than it is being absorbed. The very high light intensity you have is driving the photosynthesis. I think you should drop the 96 watt light until you get pressurized CO2, or you will soon find you are growing a bumper crop of algae.