From my planted tank days I learned that only dual stage is the way to go and everybody recommended either Matheson or Victor regulators. I've had a Matheson regulator with clippard solenoid and needle valve and I was able to fine tune the combo to as little as 1 bps
Co2-11 CarbonDoser Electronic Co2 Regulator
$249.99
Co2-LCD Electronic Co2 Regulator (with 8 digit LCD Counter)
$299.99
have one for 2-3 yrs now....working very well. i have the manual dial one. save $50 if you forgo the lcd readout. they didn't have the lcd read out one when i got mine. i just count the flashing led light.