Dual Gauge CO2 Regulator with Solenoid The Dual Gauge CO2 Regulator has an integrated Solenoid that for night time shut off to match the cycle of your lights. The most important nutrient for plant growth is carbon. When CO2 is a limiting factor, plants cannot fully utilize all the available light to photosynthesize. The introduction of carbon into an aquarium is typically achieved by using a pressurized gas tank controlled by a regulator, solenoid and needle valve. Pressurized CO2 is the best way to go for the more advanced hobbyists or for those keeping larger aquariums.