Shutting off the CO2 at night does cause a pH swing, but pH swings are a fact of life for plants in the wild, so it isn't harmful. A pH crash wont occur since the bubble rate is such that during the day the ppm of CO2 is stable and where you want it. At night it is shut off with the solenoid, so no crash then. At worst, the solenoid will fail and the pH will drop a little perhaps, due to lack of the plants using CO2 then. But, no crash. I had exactly that failure and wasn't even aware of it for some time later.

I've roasted 6 solenoids over the years FYI.
2 recently due to electrical surges.

Regards,
Tom Barr