if no CO2 escapes to the surface and dissolves completely, then pH must go down. How fast depends on CO2 amount, water quantity and KH. Usually it takes about 24 hours to get pH stabilized.
However, if your sand includes calcite or lime, then the CO2 will keep dissolving more and more KH from the sand and that will be pushing pH up. This can take a week or so.
Interesting is, when I need to get CO2 out of water, then I have to leave a cup standing on table for 24 to 48 hours to get stable pH reading.
Okay, well last night when I got home my pH still showed 7.00 so I knew something was wrong. I recalibrated my pH controller and found that I did not calibrate it right the first time and when I put it back in my tank the pH was 6.4. So I set my pH meter to 6.65 with a variance of 0.00. that way the CO2 will kick on when my pH hits 6.65 or higher and will shut off when it goes down to 6.6.64 but CO2 will continue to come out of the diffuser even after the solenoid shuts off so it brings the pH down to 6.61 then it will start to rise again.