Lowers hardness (GH) and softens water without adding harmful chemicals. Convenient pouch is rechargeable! Contains ion exchange resins to remove calcium, magnesium and soluble heavy metal ions, including iron and copper. It consists of a special “pillow” that fits into aquarium filters providing continuous removal of minerals and heavy metals. When the ion-exchange process is complete, the pouch is recharged by placing the pillow in a salt solution. Recharging the pillow reverses the ion-exchange process and makes the pouch ready for re-use. Because this process exchanges only sodium, it does not add salt to the aquarium, only harmless sodium ions. NOTE: heavy metals, like copper and iron, can permanently bind to the resin beads in the Water Softener Pillow. This can gradually reduce the efficiency of the pillow. Many fishes, such as tetras, angelfish and South American cichlids prefer the soft water common to the Amazon River Basin, (50 to 100 ppm GH, typically). The Water Softener Pillow helps to re-create the soft water found in many tropical areas of the world where aquarium fish come from. This is the “Large” pillow.