Heres the deal with the Co2 and PH. While its true, Co2 will drop your PH to some degree (usually a full point), this change is not negative in any way to your fish. This is because the hardness of your water is not changing. When fish have troubles due to PH fluctuations this is because they are experiencing what is known as osmotic shock due to a hardness change associated with the PH drop/add. The trouble with these DIY co2 units is the inability to regulate the process.

At night, plants are unable to use co2 and in fact spend the evening releasing Co2 into the water. Therefor its likely you will need to add a bubbler to your aquarium to specifically begin running the moment your lights shut off at night. If you do not do this, you may find a wiped tank in the morning not due to PH crash, but suffocation. Hope this helps.