The Nutrafin CO2 Natural Plant System Activator and Stabilizer Refill Pack is for use with the Nutrafin CO2 Natural Plant System. The Nutrafin CO2 Natural Plant System provides plants with a key nutrient that is lacking in most aquariums, Carbon Dioxide (CO2). This natural fermentation system has been designed to provide a simple and effective approach to providing dissolved CO2 to aquariums. Using a proven concept, every component has been carefully conceived to enhance performance. The level of CO2 generated by the Nutrafin CO2 Natural plant System is ideally suited to most planted aquariums illuminated with fluorescent lighting. It is important to refill your system with the Nutrafin CO2 Natural Plant System Activator and Stabilizer replacement media for proper use.