Pura (Magnavore) APU00411 Nitratelock Aquarium Filter Media for 50-Gallon Tank, 500ml Nitrate lock is unique, true nitrate remover that works 100-percent in fresh and saltwater Rapidly removes and maintains extremely low to non-measurable levels of nitrates in fresh and saltwater aquariums Features regeneration is accomplished with plain table salt quick and completely safe Drastically reduces number of necessary water changes This 500-mL Nitrate lock media comes with a 6-inch by 12-inch 300-micron media bag; treats 50-gallon tank for up to 24 months