Try running the light a little longer. Plants will raise the pH if they
are left in the dark too long. Photosynthesis takes place only when the
light is present ... in the dark plants may create a CO2 spike.

But before jumping to any conclutions, try taking about a litre of
water and adding a few drops of the plant food in it. Test the pH
before and after to see if that is playing any role.

To test the role of the plants themselves, test the water as the lights
turn on, and as they are turning off. From what I know the pH should be
higher in the morning, than it will in the evening.

Good luck with that.

Ed K.