Connected to a submersible pump or an external pump this unit ensures that water just below the surface are used first. Technically it is the oldest water that you want to use first. The slow inflow at the bottom introduces the oxygen rich water at the bottom of the tank while the floating suction pulls the old water from the top.

A course suction filter with a 150mm diameter ball

1 and a 1/4 inch hose connection without a back-flow prevention valve