Chuck, interesting question and here's my recent data because I'm struggling with the EXACT same issues. I've read Randy Holmes Farley, extensively in the Chem forum. His supposition (and I trust him -- he's ungodly smart) is that ALL PH issues are caused from C02 build up if your Alk numbers are correct.
My specific situation was that I was seeing pH as low as 7.8 overnight (!!) and barely 8 during the day. You all may remember we had several beautiful days last week (the last cool ones of the year probably) and so I threw open all the windows in the house. Within hours my pH was 8.3! I hadn't supplemented anything, nor made any other modifications. Yes, it still dropped a bit overnight (8.15ish) but during the day, bam, right back up.
It warmed back up and I closed the windows, right back down. As an experiment I ran a piece of 1/4" ice-maker tubing to the window across the room and noted after 24 hours the pH had been affected perhaps .05 units -- not significant enough.
What I've been doing the last three nights is opening a window in my living room when I go to bed (when the outside temp is <78) and my pH has been higher, though not 8.3, because the house is closed all day with two adults and four children in it all day long, creating C02.
My understanding of it is this:
#1 O2 saturation and CO2 levels are not directly related. You can be O2 saturated and still have elevated CO2. The skimmer outside air intake may help with O2 sat but this will not directly drive off CO2.
#2 In home CO2 levels will reach equilibrium with your tank water. Atmospheric CO2 is currently 387 PPM (0.0387%). I've got to believe that respiration for humans in a closed space can easily drive that number to a magnitude higher. In fact, according to the Wisconsin Department of Health (the first resource I found but you can see others on Google):
The levels of CO2 in the air and potential health problems are:
* 250 - 350 ppm â€"œ background (normal) outdoor air level
* 350- 1,000 ppm - typical level found in occupied spaces with good air exchange.
* 1,000 â€"œ 2,000 ppm - level associated with complaints of drowsiness and poor air.
* 2,000 â€"œ 5,000 ppm â€"œ level associated with headaches, sleepiness, and stagnant, stale, stuffy air. Poor concentration, loss of attention, increased heart rate and slight nausea may also be present.
* >5,000 ppm â€"œ Exposure may lead to serious oxygen deprivation resulting in permanent brain damage, coma and even death.
So you could have 1000 PPM CO2 and still be "typical" -- which means you wouldn't know or feel it -- but your tank will.
I'm considering having an outside vent put into my AC closet, too -- I think the small drop in efficiency for the AC will be worth it to have more fresh air.
Hope this helps,
Mark
