24hr & 12hr CO2
Someone posted a thread at Aquaria Central about this. I responded with my own experience and also threw in some theoretical questions, but no one responded, so I wanted to ask here:
"Just wanted to throw in a little of my own testing.
I have my CO2 on a timer. I tested pH at around 2PM (about middle time for lights & CO2 on), and it showed 6.8. Later that night (3AM) when the CO2 and lights were off, I tested the pH again and got 7.0. I only did this once so there may be a .2 +/- there, but for me, there was very little change from day to night.
I was thinking about this earlier and wondered if it has to do with what your pH and KH is out of tap. If your tank starts out with your tap water, for ex. mine's 7.2, during the night when your CO2 is off, wouldn't your tank only reach what was your tap originally (this would be the "max" it could rise)? Plants do release O2 at night (but not sure how much, though from the articles and posts I've read about it, they say it is minimal), so maybe add on .2 more pH's "max". So this would mean your tank only reaches .2 more than your tap is. In turn, this would mean the more drastic the pH swing could depend on where you start out (tap), and where you end up (CO2).
As for KH, I was thinking if it is high like say 11 (mine), would mean it would take more time for pH to rise or fall? This would mean it would be less likely that a big pH swing would occur when CO2 is shut off in those tanks that have higher KH, than those with lower KH.
These are all just theoretical ideas, but I thought I'd put them out there.
What really needs to be done is someone test their water who have nothing added in their tank that would change the water chemistry and have a planted tank with no carbon at all. They would test in the day, then in the night. Then we might know how much plants really do change water chemistry by adding oxygen and carbon."