A PH decrease of about 1.0 from degassed to whatever is in the tank, means 30-40 ppm of CO2 in the water. But, if the decrease is 1.3, those CO2 numbers double! So, obviously you have to be careful not to go more than a 1.1 decrease, or the fish will suffer. I think the best way to do this is to increase CO2 just a bit each day, until you get the 1.0 decrease, then use the bubble rate you see at this time as the bubble rate you want to maintain. If you try to control the CO2 to the right level using a PH controller, all it takes is a KH drop or a reduction in PH contribution from something else, other than CO2, in the water and you have too much CO2.