pH controllers seem to be a waste of money for added complexity. Just
measure your pH with a cheap kit-the accuracy doesn't matter that
much-it's the change in ph before and after adding CO2 that matters!

You cannot intelligently use pressurized CO2 without knowing your KH
level and your pH change.

I also echo Nikki's question-what is the reason for changing KH? Also
her recommendation: read Chuck Gadd's excellent page on CO2 at
http://www.csd.net/~cgadd/aqua/art_plant_co2chart.htm