Practical 150 Micron 4"x15" Fish Aquarium Marine Sump Felt Pre Filter Sock Bag The best form of mechanical filtration. An essential part for wet/dry filter systems, sumps, and diy filters. Used as mechanical filtration for any sumps and DIY filters. Removes excess food, detritus, organic wastes, and other particulates to lower ammonia levels and other undesirable substances. Ring material: Polypropylene