Dymax Filter Socks 7The Dymax Filter Sock is made of a highly durable material capable of capturing a large amount of debris from the water column. They are most commonly used for aquariums with an overflow, and are suitable for both freshwater and marine aquariums. Filter socks should be washed or changed regularly to maintain optimum performance. Each filter sock, has a 4" diameter ring and each order will contain 1 piece.