The Fluval A422 is the correct replacement cartridge for the following systems:
The Fluval A422 - G6 Aquarium Nitrate Filter Cartridge is a replacement cartridge for the Fluval G6 filter system that fights excess nitrate. This Fluval cartridge removes up to 24,000 milligrams of nitrate ion in freshwater aquariums. This nitrate filter keeps your aquarium free of biological waste and ensures that trapped ions will not get back into your aquarium water.
Fluval A422 - G6 Aquarium Nitrate Filter Cartridge specifications: