I've been tempted to invest in a pH controller in an attempt to keep CO2 levels stable at around 30ppm.

I've had good results with 24/7 injection before in my 33 gal. This time I'd like to save on CO2 refills (500g cylinder) and either go for a solenoid linked to the lights or a pH controller.

Any thoughts on 24/7 vs. solenoid vs. pH controller?