Black Knight filter brushes are made from the very highest quality Hydrotek polypropylene twisted into two stainless steel wires to form a cylindrical brush with a loop at one end for hanging. The other end is capped with a plastic knob for safety. They last indefinitely and are the easiest to use, most efficient and most cost-effective pond filter medium you can buy. Our brushes are used in filtration systems in fish farms, hatcheries, public aquaria, domestic fish ponds and research institutes in this country and abroad. No other filter medium is as widely used or has been as extensively tested and proved. 16" x 6" (40x15cm)