Tetra CO2 Depot Carbon dioxide fertiliser for lush aquatic plants. Replacement bottle for Tetra CO2 Optimat. Easy way of enriching the aquarium water with CO2. Aquatic plants develop stronger and healthier. The optimum growth of plants leads to an increase in the water’s oxygen content which is important for the fish’s well-being. Prevention of lime deposits on the leaves and glass panes. For aquariums containing up to 130 litres.