Put a stub of 1/2" on each lily pipe and run 5/8" from the Fluval to the pipes w/ zip ties @ the connections? My only concern is restricting the diameter of the intake on the Fluval. Maybe try it, but only run the Fluval @ 50%-60% via the aquastop. OR, better yet, get a 5/8" inline barbed hose valve (2 little fishies makes one) and use that on the return line. That way the aquastop doesn't restrict the intake even more.