With a yeast based system you can let it bubble into the tank 24/7. Excessive CO2
should my be an issue as the output is generally low. If you can concerned, you can hook up an air pump and powerhead to a timer and set it to run at night.
If you want a DIY CO2
system you can shut off, look into citric acid. Unlike yeast, it is a chemical reaction and valves are used to control the amount of reactants and amount of CO2
Sent from my iPhone using Aquarium Advice