The Salifert CO2 test gives you an accurate reading of the CO2 level present in your aquarium. A CO2 dosage is used to get the best possible result for the health and growth of your aquatic plants. If you dose CO2 in your aquarium, it is strongly recommended that you test it to make sure you have the right amount to benefit your plants. An overdose of CO2 is toxic for your fish and invertebrates.