A cassette filter for cleaning a fluid includes: a frame; and a plurality of filter elements having a filter medium provided with a folding. The plurality of filter elements are accommodated in a frame such that at least two adjoining filter elements of the plurality of filter elements are arranged at an acute angle to one another and form a V-shaped arrangement. Each of the plurality of filter elements has folded edges which are formed by fold peaks and fold valleys and extend perpendicularly to legs of the acute angle. Each filter element of the plurality of filter elements is provided with foldings such that a height of the respective filter element is smaller at all edge regions than a height in a central region of the respective filter element, and smaller folding surfaces than the central region result at the edge regions.