EHEIM CO2-SET Reagent Indicator 10mlCO2 accessories EHEIM for healthy plant growth in the aquarium. In principle, you should always keep an eye on the CO2 content of your aquarium. The pH of the water is strongly influenced by the addition of CO2. Therefore, permanent monitoring always includes a long-term CO2 test and indicator fluid as well as water test strips for a CO2 fertilization system.