Co2 in the house might be causing the shift from 7.6 to 7.9, but if you calculate an error margin it's probably only about half that. See if you can repeat the test.
Regardless, I'd bet good money that your calibration is off. I calibrated my apex pro ph probe 10 times and got 10 different baselines. There are too many variables to control the calibration process. I compared it against several different test kits and got several different results. The apex controllers are nice (I have one) but consider the cost of these things. They are cheap hobbyist equipment and are not all that accurate.
Unless you observe a problem that you believe is related to low pH, or you are running a calcium reator, then I really wouldn't worry. Just recalibrate again until you get a value that you like better. I found that resetting the apex to factory defaults fixed my low pH prob. Use the probe to understand the fluctuations in your tank, not to determine a precise value. Also useful for preventing spikes.
- Brad
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