I needed to reduce the internal diameter of my fish pond pump outflow port from 22 mm to 12 mm to suit the existing hose setup so I designed this reducing fitting. You can change the diameters in the OpenSCAD to suit your own installation. My first OpenSCAD to STL project :-)

Printed on a Replicator 1 using 1.70 mm ABS at 100% infill (watertight ...) with 0.2 mm layer thickness and 50 mm/s feed rate. Use a raft and print with the larger diameter sitting flush with the build plate.