It was interesting to read that the CO2 release by the plants at night would lower ph. I hadn't thought about that.
Technically, it kind of works in reverse
Fish, plants and aerobic bacteria consume oxygen and give off CO2 continually, day and night. In fish and plants we of course call it respiration, and with most fish this slows down at night due to inactivity.
During daylight, plants photosynthesize (provided they have what they need, namely adequate light and nutrients) and during this process they consume CO2 and release oxygen; this is a different process from the normal respiration which continues as I said. This photosynthesis process is much quicker than respiration, so CO2 levels in the water lower and the amount of carbonic acid produced consequently lessens. At the same time, the oxygen released will bind with organics and minerals. The combined effect is the rise in pH as the water becomes less acidic and more alkaline.
If the CO2 is exhausted but everything else (light and other nutrients) are still available, plants then look for other sources of carbon, primarily from bicarbonates. The hardness of the water determines the level of bicarbonates, and as these are used up the water becomes softer. This results in the somewhat unique situation of soft water with high pH.
During darkness, the CO2 begins to increase and oxygen is used up via respiration. The CO2 binds with minerals such as calcium, forming bicarbonates. Water hardness thus rises again. CO2 also binds with organics, creating carbonic acids again, thereby lowering the pH.
This diurnal process occurs in all natural waters. In the aquarium, where there are more plants within less volume being a closed system, it is somewhat greater than it would normally be in nature. Provided the aquarium is biologically balanced, there is no harm to the fish. A pH fluctuation of .5 over this diurnal period is harmless. Some writers say up to 1 full degree is not an issue, though the fluctuation is unlikely to be that significant in most aquaria.
Adding CO2 via diffusion has an impact on this diurnal fluctuation, and it is possible to regulate this fluctuation. Providing additional CO2 to plants during daylight means they do not need to use bicarbonates, thus reducing the fluctuations in hardness. The pH fluctuations still occur though, and if the CO2 is left running during darkness it can increase CO2 to such an extent that the fish do suffer. Not only from the CO2, but also the more dramatic shift in pH that will occur. The CO2 diffusion should be timed to shut off with the lights.