You could use a solenoid to open when the pH get down to a specific level. Have the Solenoid teed off the CO2 line that goes into the tank. When the solenoid opens, the CO2 bleeds out through the solenoid instead of going into the tank. You won't be using any less CO2, but if, without the solenoid, it's dumping more than is desired, you won't be wasting any *more* -- and you won't be hurting the fish.
***************************** Waveform Technology UNIX Systems Administrator
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