In my experience, unless you are keeping a low KH there is really no
need to turn CO2 off at night. In most tanks, the biggest loss of CO2
is not presented by the plants, it's from gas exchange, and this
doesn't stop at night. Therefore the impact the plants have on CO2
levels during the day is really not as big as some may think it is.
Obviously if you have no surface agitation you may see a totally
different picture, but then you would likely have oxygen problems
during the night regardless of CO2 injection. Fish load will also have
an impact on oxygen levels during the night and if gas exchange is
insufficient you may run into problems if overstocked. Personally I
never have, I used to turn off CO2 at night when I kept a very low KH,
and a PH controller would be wonderful in those situations, but with a
KH of around 3-4 or higher you shouldn't have any problems with PH
swings during the night from CO2.

If there's one thing I've learnied that is to not trust everything you
read in books or hear from others, try it out for yourself and you
will have a better idea, and even then, your conclusions won't work
for every tank.

Now having said that, what does this have to do with a CO2 regulator?
;-)

Regards
Giancarlo Podio