Hello,
It sounds like the setting for the CO2 rate was too high, and overtime too much CO2 was accumulated within the system. Have CO2 re-circulation prevents pump from cavitating but mis-tuning can lead to the build up of more and more CO2 overtime. Try opening up your effluent valve all the way to drain as much CO2 as possible and increase internal pH, which allow more CO2 to be dissolved at the same time. Do this for say a minute or so and see if it helps to correct the problem. Then also slow down the CO2 rate.
The rise of pH appears to come from the slower turnover by the pump from a lowered efficiency (because of bubbles).
Titus
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