Water filters which comprise a multitude of discrete filter media elements in a chamber require periodic cleaning. However, the elements tend to become adhered to one another and to the interior surface of the chamber. The invention provides a water filter for use with ponds, comprising a filter chamber for containing filter media, and a first movable cutting member provided substantially immediately adjacent the interior wall of the chamber for movement across the surface of the interior surface of the wall to break the adherence of filter media to the interior wall.

A filtration apparatus includes: a filtration tank; a layer of filter medium disposed in an interior of the filtration tank, the layer of filter medium being used to pass supplied raw water therethrough from bottom to top to filter the raw water; and a filter medium washing device having a helical screw placed upright in the layer of filter medium and a screw driving unit configured to rotate the screw about a screw axis, the filter medium washing device being configured to rotate the screw to scrub and wash the filter medium. The filter medium washing device, which does not include an outer cylinder surrounding the screw, is further configured to transport the filter medium upward in a state in which an outer peripheral edge of the screw is in direct contact with the layer of filter medium.