Boyd Chemi-Bags make it easy to use the many types of granular and resin filtration media available today. The bags are made of durable flow-through nylon mesh. The mesh is small enough to hold tiny resin beads but still allow water to pass through the bag and filter media. Chemi-Bags are double-stitched and tough enough to pack with activated carbon, granular ammonia, nitrate, phosphate and silicate removers. You can even create your own custom blend of filter media. Chemi-Bags measure 8.5 x 4.5 inches and fit into any aquarium filter or sump. The bags come in a 2-pack and include 2 nylon ties.