OK thanks that makes sense. I use a pH controller to control the amount of CO2.
I also have a gas flow meter/adjuster that I use in place of the needle valve. Interestingly the flow reading was never useful when I just bubbled into the canister intake... I had tweak it down to an indicated 0 to get the bubble rate down to 2/sec. Perhaps the low pressure from the venturi effect off the intake flow was throwing it off. Now that I have the diffuser connected which definitely needs some pressure to diffuse CO2, it reads ~15ml/min for 2 bubbles/sec, which seems higher than I would expect, but I can't find any leaks. I have CO2 tubing from the tank to the check valve, and silicone tubing from the check valve to the diffuser.
Tim