Well, if the pH was constant, the CO2 level was the same as normal.
One possiblity: I don't know how much CO2 the tank loses due to surface
agitation etc. A powerhead isn't an efficient diffusor, some co2 bubbles
might remain that will eventually reach the surface. If the airspace
over the tank is fairly well sealed, then the CO2 could displace all the
air over the tank. Then the tank O2 level would drop, since the main
source of O2 for the tank is the water-air interface. I've seen this
-- Chuck Gadd