Plants only absorb CO2 when your lights are on, so there is no point in adding gas at night time. Not only is it wasteful, but carbon dioxide binds to oxygen and you can cause major distress or death in your livestock.
The Colombo CO2 Solenoid should be used with a domestic timer (just like your lights timer) and automates when your CO2 comes on and off. I recommend it comes on 1 hour before your lights come on and turn off 1 hour before they go off (this is optimal). By using this solenoid you will not only save a lot of gas, reduce any risk to your fish/shrimp and save you plenty of money (as you won't need to purchase CO2 bottles as often!).