Plants do not always up take co2, just during the photo period. At night they take in oxygen and give off co2. So, with out a solenoid you run the risk of killing your fish from a lack of oxygen if the co2 is on. I guess the solenoid is not a necessity, if you plant not to have any fish. It was once put to me this way, if your co2 is efficient and on at night you fish will die.