An accumulating tank serving as a filter for an overflowing swimming pool, and more particularly a device for filtering water from an overflowing swimming pool using an accumulating tank acting as a gravity filter. The accumulating tank (1) serves as a prefilter, and a filter for water from the pool; it filters by decantation through a filtering mass (8, 9). It comprises apparatus for cleaning the filtering mass (8, 9) by reversing the direction of water current (F2), so as to cause to overflow the volume of water from the accumulating tank (1) and recover the impurities by overflow of the used water into a gutter (15) and evacuation for emptying by a pipe (24).