phosfiltrum is a specially formulated, high-density, granular ferric oxide (GFO). It will rapidly remove phosphates and silicates from both freshwater and saltwater and will not release them back into your aquarium. It is much harder and consequently much less fragile than other GFOs, so it will not break apart into fine particles in your tank. It has the highest binding capacity of any phosphate remover on the market, so it will last much longer before it needs to be replaced. Its distinctive dark brown color identifies it as the premier product in the industry.