This is a 3D printable solution for your aquarium pump return line - it was designed for a sump-in-tank design where the wall of your sump is ~2mm thick (although thicker should also work) and a pump using a ½" (~12mm) ID return line. If your tubing is a different internal diameter this may work, or you may require a different adapter for the connector piece, but the remainder of the system is largely pump- and tubing-agnostic.

Print everything with the obvious sides down, you will need to enable support for the Rear mount/Line Connector, Diffusion End, and Flex Mount pieces. I recommend printing in the smallest layer height you have available, this will make the flexible pieces seal better and move more easily as well as make the printed threads mesh more easily.

Be careful when assembling, it's easy to overtorque and break the threads, especially on the Flex Mount piece.

You can print this in any of the common filaments, PLA (or PLA+), ABS/ASA, PETG, or Nylon, all of them are reef safe and will stand up to fresh or salt water conditions. I made mine in PLA+ and the flex pieces fit together with some effort and remain water tight even under pressure.