Tom Barr has pointed out that natural lakes can swing two full pH points
from
day to night without harming the fish. If you are concerned, run the CO2
24
hours per day. It will tend to swing a bit up during the day, a bit down
at
night, probably just 0.2 or so. I did detailed testing in my tank a year
ago and
found that the swing was less than 0.2.

And also, doesn't it depend somewhat on the strength or quality/efficiency
of your lighting? because if the lights are not strong, then the
photosynthesis that takes place is shorter in length and the ph doesn't drop
as much?

I'm probably way off... but seem to remember someone talking about that last
year when I got my lights and CO2 at the same time... they said put the
lights on, the slowly add the Co2 to monitor to watch for the daily swing...
I got .5 of a swing, and was told that anything less then 2 would be ok....