Aquarium Supplies Water Softener Pillow ION Exchange - GH Reducer Filter Water Softener Pillow (250ml / 8.45 fl oz.) x1 .Exchanges cations with resin. It effectively removes calcium and magnesium cations Mechanical filter medium for freshwater aquariums. Softens water and maintains a stable pH level. Suitable for freshwater fish such as discus, tetra, cichlids, blood parrot,killifish and betta. Provides the ideal environment for plant ecology. Water Softener Pillow is a universal product and can be placed in most types of aquarium filter systems.