If, before you added CO2 your pH was 7.4 and your KH was 4, it would give you a reading of 5 mg/l of CO2. Which I guess, is probably the reason you decided to add CO2. However, if you are saying your pH is now 6.0 and you were still to have a KH of 4, this would give you 133mg/l CO2 ( too...