Rather like Matt, I had an epiphany moment this morning, although mine was when I was driving to work.... I'd been thinking about getting some CO2 in my tank as it seems all the rage at the moment, and was wondering about DIY/yeast/FE's etc. The general consensus seems to be that you need stable CO2 and to achieve this, you should do away with pH controllers/solenoids and inject 24/7. But then, I thought, the CO2 concentration would go up at night as the plants wouldn't be using it and then it would come down in the morning when the lights were on. That doesn't sound very stable. But pH controllers effectively do provide a stable CO2 as the pH is related to CO2 concentration. Discuss!!