The Fluval A421 is a nitrate adsorbing filter cartridge that is for use in the following aquarium filtation system:
The Fluval A421 - G3 Nitrate Filter Cartridge removes up to 13000 mg of nitrate ion in freshwater aquariums. This Fluval G3 replacement helps even out the nitrate level in your freshwater aquarium. Nitrate concentration should not exceed 20 mg/L in freshwater aquariums. Planted aquariums require approximately 5 mg/L of nitrate as a source of nitrogen for healthy plant growth. This nitrate filter features a long-lasting large capacity ion exchange resin.
The Fluval A421 specifications: