pH keeps drifting up overnight. What am I doing wrong?

Running a small Kratky setup with lettuce. I set the pH to 5.8 before bed and by morning it is sitting at 7.2 or higher. I have tried different pH down products and the result is the same. Using tap water that sits at about 7.6 out of the tap. The EC is fine. Is this a water quality issue or something else?