I think if you are running CO2, then a KH and PH test kits should be standard equipment. Even if you use a drop checker for CO2 monitoring instead, a KH test is good for verifying the water used in the drop checker.
For my DIY CO2, I'm running two 2-liter bottles, tee into one bubble counter (with built-in check valve), into hole drilled in Fluval FX5 intake. I use a Red Sea drop checker for monitoring.