The inlet pipe allows the unit to be primed for the initial filter startup and if the filter needs to be re-primed after a major cleaning or tear down and reset. Once primed, water from the aquarium will be drawn through the intake strainer into the inlet pipe and out of the aquarium. This water flows into the canister and should fill the canister to the top. Just restart as normal. This is version two but it will also work with the original accessories. This version includes the filler cap.