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pH Variations

After more than a year of fiddling with the needle valve on my CO2 system,
trying to achieve a steady pH reading, I recently purchased a Milwaukee
SMS 122 pH monitor/controller. After calibrating it, I set it at pH = 6.6,
which at my KH of 4.0, should provide a CO2 level of 30 ppm.

It works as it should and keeps a nice steady reading of 6.6 -- as long as
the lights are on.  It shouldn't have been a surprise to me, that the pH
would drop during the dark hours, but I didn't realize by how much. For
the next two hours, as the plants produce CO2, the reading slowly falls to
6.4 and stays there until the lights come on again the next day.  Then,
when the lights come on again, and the plants begin to use CO2, the pH
slowly rises for about two hours, until the monitor/controller kicks in
and turns on the gas.

According to the chart, a pH of 6.4 corresponds to a CO2 level of 47 ppm,
which I have read is too high for fish.  On the other hand, this must have
been going on long before I had a way of monitoring the pH level so
easily, and I have never detected any ill effect on the fish (15 Cardinal
tetras, 5 Harlequin rasboras, 2 oto cats, 1 SAE., and 1 clown pleco).

So my question is: Do I have a problem?  Should I raise the light period
pH setting or will the level probably fall to 6.4 at night anyway?  That
seems to be some kind of plateau, because I have checked it at 2 a.m. and
5:30 a.m. as well as at 11:45 a.m. when the lights come on, and it is
always 6.4.  Thanks in advance for your opinions and/or advice.

John T. Fitch
E-mail: JTFitch at FitchFamily_com
Web Site: www.fitchfamily.com/aquarium.html