Phos-Zorb is a special filtration material that selectively removes phosphate and silicate. Phosphate is a by-product of fish metabolism, decaying organic matter, and is an additive in some water conditioners and filter medias. In saltwater aquariums, excess phosphate lowers the available calcium and magnesium necessary for the growth of hard corals and other invertebrates. Comes in one 6 1/2" x 6 1/2" pouch. One pouch treats 55 gallons. Place pouch directly above foam filter pads. Replace pouches every 1-3 months.