Conventional long-term CO2 tests are filled with a mixture of aquarium water and indicator. This includes various disadvantages, one of which is that the acids present in the aquarium water (e.g. humic acids, nitrates, etc.) can lead to incorrect test results. The long-term DENNERLE CO2 Correct test uses a ready-to-use special CO2 indicator that is used on its own and not in combination with aquarium water. No additional measurement of carbonate hardness is required. Only a color matching scale is needed.