both will result in pH changes. I asked the same question, too, and was told basically not to worry. Reassured that changes <1.0 pH in a day are tolerable to fish. They're natural pH fluctuations as the result of CO2 reacting with water to form carbonic acid (CO2 + H20 = H3CO2). They happen in nature and are much preferable to rapid swings due to chemical pH adjusters.
So when you shut off CO2, your pH will increase (get more basic). When you inject CO2 without lights, the plants are not photosynthesizing so they will not absorb excess carbon dioxide gas. In this case, the pH change toward acidity will be more drastic.
This last point, in conjunction with the fact that plants need oxygen at night (in fact compete with animals for O2 in the absence of light) means that you should either turn off CO2 at night, or dissipate it with an airstone.
So when you shut off CO2, your pH will increase (get more basic). When you inject CO2 without lights, the plants are not photosynthesizing so they will not absorb excess carbon dioxide gas. In this case, the pH change toward acidity will be more drastic.
This last point, in conjunction with the fact that plants need oxygen at night (in fact compete with animals for O2 in the absence of light) means that you should either turn off CO2 at night, or dissipate it with an airstone.
