NO2 (Nitrite) & NO3 (Nitrate) are 2 toxic and harmful substances to all fishes Fish red blood cells with nitrite present lose the ability to transport oxygen. When Nitrate level becomes too high, it triggers the burst out of algae problem To marine tank, high NO3 can badly effects corals absorbing element Ca to build skeleton NO2 & NO3 Minus Media with professional resin spray coated to effectively remove Nitrite and Nitrate Simply place NO2 & NO3 Minus Media in your filtering system to remove Nitrite and Nitrate 1 piece to treat 100L of water, for very dirty aquariums, you should add 2-3 more pieces to clean your water