CO2 releases Carbonic Acids, and acids lower PH. The cool thing about CO2 in fish tanks is that it lowers PH and it seems that it sidesteps the buffer. In other words, the buffer has no effect on it but the buffer also remains intact. So when the PH is lowered it is also stable. But, When the lights are off plant do not take in anymore CO2. So if you are using a DIY CO2 you would have to run a bubbler at night on a timer. But that also depends on your stock and how planted your tank is. I have ran DIY CO2 with no bubbler because my tank was moderately planted and the tank was not overstocked, while stocked with fish on the smaller side.
