If you're tralking about turning the CO2 completely off at night when the lights are off... depending on surface agitation, CO2 concentration and your KH, pH can "swing wildly"
if you turn off the CO2 at night. I run my CO2 controller 24x7 - lights on or off - specifically to keep the pH stable.
If you're talking about the solenoid turning off the CO2 because the pH controller told it to, then it will depend on the "dead band" of your controller. I have mine set at .05 pH. That means if I have my controller set at, say 6.9 pH, the controller turns on the CO2 at 6.9 and shuts off at 6.85. If your dead band is set too wide, you can get a significant up and down fluctuation in pH as the controller turns the CO2 on and off.
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