These are designed to plump the tank i.e. these bits fit around the hole in your tank, worth noting the sizes quoted are the pipe that they are designed for. For example the 32mm fitting will take 32mm pipe and requires a hole of at least 43mm, if in any doubt please contact us. Inside/outside diameters for these fittings are: 20mm/27mm - 25mm/34mm - 32mm/43mm - 40mm/55mm - 50mm/61mm - 63mm/77mm. PLEASE NOTE all our pipe is fully fish safe and has been used by us for many years, all joints are glue fittings using standard 'solvent weld' glue available from us or most DIY shops. All our pipework is metric and directly compatible with all European pipe such as VDL, it is also directly compatible with Aqua Medic Deltec Surchan and almost all European equipment. It will not fit directly onto most U.K. sourced pipework without a metric to imperial adaptor. If you are in any doubt please contact us for advice.