Abstract: A rain water run-off filtering system includes a tank for holding and filtering run-off water, said tank being generally cylindrical with a horizontal axis, and having at least one manway for the introduction of water and a filter media. A horizontal grate disposed at the bottom of the tank, supports a bed of gravel and sand, and incoming water is directed onto the sand bed, thence percolating through the sand to a space below the grate. The outflow of water from the tank is regulated by a baffle plate bounding the sand bed. This plate has small holes for normally producing a small outflow, and at least one large aperture for permitting water to flow through the tank, without percolating through the sand, during extremely heavy rains.