The ISTA Replacement Ceramic for External CO2 is made to be used in the Ista External CO2 Ceramic Reactor - Item #IS00540. Specialized ceramic is used to ensure proper diffusion regardless of CO2 flow rate, it is also very easily cleaned and replaced! Replacing it ensures that the diffuser continues to produce very small CO2 bubbles, which optimizes the diffusion of the gas in the aquarium water.