The Tetra CO2 Optimat Depot is a simple way to add additional carbon dioxide to the aquarium, and thereby supporting healthy, lush plant growth. How it Works:The CO2 Optimat delivers concentrated carbon dioxide to a diffuser in the aquarium. Here it slowly diffuses into the aquarium during the course of the day, increasing the carbon dioxide level and promoting good plant growth. How to Use:Simply press the top of the CO2 Optimat to squirt carbon dioxide into the diffuser. These needs to be done once or twice a day, depending on aquarium conditions. Regular checking of the carbon dioxide level with a TetraTest Laborett kit is recommended, to ensure that the correct concentrations are being achieved. Ideal for tanks up to 100 litres