filling material for percolating filters used in the biological treatment of fluids are provided with strips which are made of plastic and are connected to one another via a transversely directed holding web . the strips are arranged one above the other in a plurality of layers in order to increase the strip density . in the filling material according to the invention , the strips are provided with different cross - sections over their length . for example , the strips may be reduced in width in regions adjacent to their connection to the holding web . as a result , the strips can be arranged in one layer with an increased strip density .