C02 and pH swing

I just started a C02 setup with a bottle. I've closely monitored it
for two days and it looks like this:

Initial parameters:
pH 7.4
KH 40 ppm
C02 calc 2.6

At the end of the first day, in the evening:
pH 6.7
KH 40 ppm
C02 calc 13.1

5:00 next morning (C02 is off at night, 1 hour after lights out)
pH 7.0

Later that day pH dropped down to 6.6 for a C02 calc of 16.5

Is a swing from 7.1 to 6.6 on a daily basis what others see? Should I
keep the C02 on all night long? How much lower will I expect the pH to
drop if I leave it on all night while the plants don't eat it?
thanks,
steve