The inlet should be as far from the outlet as possible, and at a
different level in the tank. This helps encourage circulation.
Popular configurations are a fairly low inlet with a spraybar outlet at
the top of the tank, or an overflow inlet with an outlet deep in the
tank. Overflows are good for planted tanks where a surface film is
common and you want to catch bits of leaves and floating debris in the
prefilter. Spraybars with a low inlet are good for tanks with a lot of
fish or large fish. The low inlet is better at sucking debris out of
the tank and the spraybar provides extra oxygenation.