Ista Glass CO2 Diffuser is for dissolving carbon dioxide (CO2) into planted aquariums. Hand made from high-quality glass this model fits in-line into the return pipe of the external filter. A piece of 4/6mm CO2 proof hose from the CO2 fertilization system is then attached to the base of the unit. As the water passes the CO2 dissolves into the water. 2 models available to suit aquariums up to 200 and 500 litres.