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 45g CO2 Disposable Cartridges are available in a pack of 3. These replacement cartridges are suitable for the Fluval Pressurized 45g CO2 Kit (17554). Cartridges are also available in a pack of 1 (17555, sold separately).