0.625 Cylinder (Jili Online Aquarium CO2 Tank Cylinder Adapter Converter Bolt Nut for Co2 Regulator, W21.8 to G5/8 ,G5/8 to W21.8, M22x1.5 to G5/8 , G5/8 to M22x1.5 - Silver, G5/8 to M22x1.5)