DYMAX CO2 IndicatorCO2 is one of the key elements for plant growth. Insufficient CO2 will result to improper growth of aquatic plants. So it is important to constantly monitor and ensure the correct amount of CO2 is supplied into the planted aquarium. The DYMAX CO2 Crystal Indicator works with an indicating solution to provide accurate reading of co2 dissolved in the water, available in two sizes. CO2 Indicator solution is sold separately.