Everyone has already been so helpful. So I purchased a Red Sea 350 that already has part #42221 (16mm) connected to the soft tubing. I want to keep everything as is. The issue I have been having is connecting my VarioS 6 return pump to the soft tubing. I ordered another part #42221 but it is just a bit too small to connect directly to the VarioS 6. I'm just getting into the hobby and know little to nothing about plumbing. Any suggestions on how I can get everything connected?