ST International CERAMIC REPLACEMENT DISC For Aquarium CO2 Atomizers Replacement ceramic disc for CO2 Atomizers Available in 20mm or 26mm sizes 20mm Disc will fit ST CO2 Atomizer 3-in-1 20mm; ST CO2 Atomizer 2-in-1 20mm; ST CO2 Atomizer 4-in-1 26mm Disc will fit ST CO2 Atomizer 2-in-1 26mm