AquariumPlants.com's Regulator

This is the regulator I use, which comes highly recommended.  It costs about $180 (free shipping when I bought it), and comes with a three year warranty.  It's flashing red light indicates when each "bubble" heads to the calcium reactor.  The electronic CO2 regulator's black box does the work, eliminating the need for a bubble counter.  You can dial the rate from 10 bubbles per second to one every 10 seconds.