pH is directly related to how the carx media is broken down. If you are expecting 25dkh effluent with a pH of 6.5, but are getting 50 plus. The pH must not be correct and actually lower.
I don't think this is the reason why air is building up. I think it is correct to assume that the co2 is building up in the reactor because the co2 is coming in too fast. My guess is something is wrong with your pH probe.
The reason why i ask is you can get more into focus (increased depth of field) with a smaller aperture (larger f-stop number). To do this you need a tripod which you seem to have.