The solenoid turns his CO2
off at night - plugged into a timer.
Otherwise you are right a pH controller would also make good use of the solenoid.
Many folks out there do not use controllers and are able to balance their CO2
injection with BPS (Bubble Per Second) tweaking with a needle valve. Personally I found that too hard and just use a controller - when on it just blasts CO2
in there until the pH is corrected and then stops. I also have the whole thing turn off at night.
And in addition to the controller I use a drop checker with reference solution to confirm that my CO2
levels are correct regardless of what my controller thinks (great first indication that your CO2
tank is empty is seeing your drop checker go blue).
A controller is going to cost you around $100 if you want to go that route... Without one, you do run a higher risk of killing all of your fish, especially if you are new to CO2
and not around your tanks for long periods of time.