6.8 during the day, 6.6 in the AM before the lights come on..

My PH fluctuates from 6.65 to 6.75 with the controler-- which is really not
that much less variance than what you have been able to achieve without a
controler. Are you able to set your flow rate and let it go for a while
without fear, or do your really need to check it often during a day? How
long does it take you to get re-set after swapping out your CO2 cylinder?

Possibly. In a tank with the absolute minimum CO2 loss, it's
reasonable to expect slightly higher CO2 accumulation during the
night.

I don't know that it's really necessary to maintain extremely stable
CO2 levels. In nature, it's common for nighttime CO2 levels to be
higher than daytime levels. This is a natural swing. In fact, this
natural CO2 swing might even be beneficial to plant growth. Here's a
study that found improved growth when CO2 levels were allowed to drift
higher at night:

http://www.co2science.org/journal/2002/v5n4b1.htm

Interesting article. Thanks. (I am amazed there is journal called "CO2
Science", glad to know about it.)

A .2 variation is nothing! I don't know that I ever succeeded in getting
that little bit of a swing without a controller. I think the best I ever
did was about .5 swing, which I thought might start killing fish (especially
since that is the BEST I ever did).

Well if I ever re-do my CO2 system, I may try getting a wet/dry filter and
(probably more importantly) a better CO2 regulator/needle valve. I have
trouble at times getting a consistent flow through my regulator/needle valve
combination, particularly after I change the CO2 tank -Bruce Geist