This is the Hagen Fluval Replacement CO2 Cartridge for CO2 20
It comes in a 3 pack of 7oz (20g) sizes. Replace these to maintain the water in your tank to be suitable for plants. We carry the kit that contains all of the necessary parts. You can find that in this category. It is a good idea to get additional cartridges since they do need occasional changing out. This way you can cut down on the time and shipping cost.