Poly filters work in power filters, undergravel filters and canister filter in both fresh and saltwater aquariums. Removes harmful organics, toxic ammonia, heavy metals, all forms of phosphates, and medications after treatment. General maintenance comments: Poly Filter needs no other chemicals, resins, carbon or other additives in fresh and marine systems to aid in filtrations Poly Filter turns color as it absorbs harmful organics, progressing from white to green to brown and then to black (time to replace). Poly Filter can be cut to any size or shape and also can be rolled into a cirular configuration. Poly Filter turns color when removing medications (blue with copper, red with iron, green with ammonia). Product will not soften water or remove trace minerals. Size: 4" x 8"