Paul wrote:

Checking the CO2 in mid afternoon I find that pH is 7.6 and KH is 10dkh =
7.5 ppm CO2, in the morning after lights on for a couple hours it checks, pH
7.0, KH 9dkh, CO2 = 27 ppm. Is this depletion good, bad or indifferent.
Should I be looking at decreasing my surface agitation of the power filter?
Thanks.

Paul,

It is unlikely that KH would swing 1 point in a few hours. I am guessing that
you are measuring pH and KH, then calculating CO2 from them. If so,
inaccuracies in the KH measurement are causing wide apparent CO2 swings.

Is your test kit fairly new? Mine allows doubling the accuracy by doubling the
amount of water. Does yours?