I am currently having a 120gal acrylic tank built. I am having the overflow centered on the back wall. I have 2 options to plumb the return. Option 1 is have the return line come back up through the over flow with a Y and click lock off the overflow. Option 2 is to have the tank drilled half way between the overflow and the end of the tank on each side. Not really sure if it makes a difference.