The Arcadia Polyfilter (Previously Underworld) is a unique, specially patented filter medium for chemically filtering and purifying both fresh and saltwater aquaria. PolyFilter will remove excess nutrients, toxins and unwanted pollutants. Also ideal for copper removal from marine tanks. The Standard/Breeder size can be cut up to fit in external / internal filters or simply placed in the sump tank. Polyfilters remove the following: Ammonia Nitrite Nitrate Copper Phosphates Antibiotics Dyes Proteins Toxic Metals Insecticides We recommend that all fish keepers use one of these in their tanks, polyfilters are simply unrivaled at removing harmful toxins and chemicals from aquarium water