South Ocean Five AOF28170 Felt Filter Media Bag for Aquarium Filter, 4 by 6-1/2-Inch Felt filter media bag is combination of felt material and polyester mesh material sewn together Made from 100-percent polyester with a polyester tie cord, sewn with a five-string merrow machine with a full safety stitch inside This gives the bags added strength for holding carbon or other substrates without stretching as well as filtering the water Using this bag in the outflow of most protein skimmers, will avoid bubbles in the sump Measures 4-inch width by 6-1/2-inch height