AquaCity Universal Filter Media Bags for Pelletized Carbon, Bio Balls, Ceramic Rings, Ammonia Remover (Black, 12.5"x 10.75" 3-Pack) Zipper design, reusable; great for use in canister filter system. Mesh holes diameter: 2mm (0.08"). Any particles less than 2mm (0.08") will pass through the bag. This bag is good for holding large media items, such as Pellet carbon, bio balls, bio ceramic rings, lava rocks, zeolite, gravel. NOT for fine, granulated charcoal media. *It is NOT intended for use with smaller particle media, such as Seachem Purigen, Seachem Phosguard, Seachem Gel Mix, Marineland Diamond-Blend Carbon/Ammonia Crystal Mix, BRX Rox, Fluval branded activated carbon, GFO/GAC, etc. Above brand name charcoal media will escape from the bag. Bag size: 12.5" x 10.75" Bag material: nylon. Zipper material: metal Ideal for use in freshwater tank.