I'm assuming you already know how to set up the reactors so I'll skip that unless you say otherwise. As far as CO2 fluctuation, you can't simply plug the solenoid into a timer to shut it off at night but what you can do is run an airstone at lights-off to strip the CO2 out of the water. I will say though that several years ago when I was running DIY CO2, I don't recall running an airstone at night and I don't remember any major issues caused by the pH swings. The important thing would be to check the fish at lights-on to make sure they're not gasping for air. Plants may consume CO2 during the day while producing oxygen, but at night they consume oxygen and may remove too much oxygen at night making it difficult for the fish to breathe.