Permanent CO2 tests, also called drop checkers, are useful tools for the long-term monitoring of CO2 in the aquarium water. In aquascapes in particular and in planted tanks in general, CO2 levels ought to be between 20 and 30 mg/l in order to guarantee optimal plant growth. Especially designed permanent CO2 test solutions are filled into the drop checker. With their help, the CO2 content of the water can be monitored, as they change colour when certain levels of CO2 are reached. CO2 monitoring is a bit delayed, though, the drop checker needs approximately one to two hours until it shows the exact CO2 level of your tank water.
Permanent CO2 tests have the advantage of measuring CO2 concentrations independently from the aquarium water. Therefore, no other substances dissolved in the water like e.g. humic acids can interfere with the test results.
Quite often, drop checkers are made of glass, which makes them almost invisible in an aquascape. However, there are also permanent CO2 test housings made of white plastic, which enable the user to see the colour change better. However, they do not look as nice as those made of glass.