Abstract: A water storage outer tank (1) has an opening (9) in the top (5) and an opening (10) in the bottom (6). An open topped elongated diverter tank (13) with an open top and a discharge opening (17) is housed within the outer tank (1) with a discharge opening (17) sealingly mounted over the outer tank bottom opening (10) and the open top of the diverter tank (13) aligned with the tank opening (9). A divider (24) with a valve seating opening (25) divides the diverter (13) into an upper zone and a lower zone which houses a buoyant float valve (26). Openings (23) upstream of the divider (24) pass water from the diverter upper zone to the interior of the tank (1) when the diverter tank lower zone is filled with water to the extent that the float valve (26) seals off the opening (25).