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pH and CO2

I have been puzzled by an apparent rise in pH at night (as indicated by a
Dennerle Visual Indicator)..  I decided to check what was happening with my
regular test kits.  Yesterday morning, I measured a pH of 7.2 and a KH of 6.
From the graph, I derived a CO2 concentration of 12 ppm.  Deciding that was
a bit low, I increased the CO2 bubble rate -- too much as it turned out --
so that by the time the lights and CO2 were turned off last ngiht, the pH
had dropped to 6.8, indicating a CO2 conc. of 28.  But this morning, the pH
was back up to 7.0.  And, thus, my question: If the plants stop absorbing
CO2 at night and respire it instead, why doesn't the pH go down, rather than

A puzzled beginner,
John T. Fitch
E-mail: JTFitch at FitchFamily_com
Web Page: www.fitchfamily.com