Since all planted aquariums lack CO2, it is extremely important that they are supplemented with it. In fact, 40 to 50% of a plant's dry weight is composed of carbon. For optimal condition of aquarium plants, some CO2 supplementation is necessary. The Fluval Ceramic CO2 Replacement Diffuser Disc is a genuine replacement disc for the Fluval Ceramic CO2 Diffuser (17548), which effectively dissolves CO2 into the aquarium.