Well what I do with my DIY C02 setup is unscrew the cap on the top of the bottle after I turn the lights off at night on the tank. Cutting the C02 really does help keep the PH in my tank stable. I have run tests to confirm this...
My understanding for this is, the plants do not use the C02 in the tank at night, therefore, the C02 left behind takes its time to disapate from the tank water. (I'm going to leave KH out of this to keep things simple)
For example, my PH at the end of my light cycle is 6.2, in the morning before the lights come on it sits at around 6.4. I tested the tank over the course of a week and readings averaged out around the same, and never a variance of more than .1
My tank is 300L and is heavery planted, so this will have an impact on the outcome of my results. However, I'm pretty happy with DIY at the moment.
Cheers,
F