I tested the pH this morning 30 minutes before lights on (and powerhead on) and the pH only moved up 0.1 so I have decided to put an airstone on a timer that will switch on for about 4 hours after lights out in order to degass the CO2. Many of you have done this without any fish harm so I might as well do it also. I just don't like the idea of high CO2 concentrations (30 ppm+) at night when the plants aren't using it.