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 45 g CO2 Disposable Cartridge is available in a pack of 1. This replacement cartridge is suitable the Fluval Pressurized 45g CO2 Kit (17554). Cartridges are also available in a pack of 3 (17556, sold separately).