I have my solenoid hooked up to the same timer as my lights. I always understood that if your lights are not running then you plants are not using the CO2, so you would be wasting it. The old photosynthesis, turning CO2 into O2 using sunlight. When I first set up my tank I kept it on 24/7 and came down one morning to my fish sucking the surface do to lack of O2. Didnt lose any but was still scary. Since then i have had it shut off with my lights and no problems.