I'm guessing most people use one of those in tank CO2 indicators. I would've thought that this water would be a more common item sold? I don't have the gram measuring scale that it would take, and a really accurate test kit for KH seems difficult to come by. I could just mix some up and test with my kit, dilute until the reading is 4dKH, as innacurate as that may be. Is that what most people do? Right now I'm just going by pH. I'm setting the controller to drop .2 pH to give me about 20ppm CO2 ...