These bulkheads are designed to create a watertight seal wherever you need a pipe to pass through the tanks glass. They are supplied with a rubber and plastic sealing ring and require no extra sealant. Inside diameters/hole in glass diameters for these fittings are: 20mm/27mm - 25mm/34mm - 32mm/43mm - 40mm/55mm - 50mm/61mm - 63mm/77mm. The glass hole diameters are the absolute minimum size.