The Fluval A420 is the correct replacement filter for the following systems:
The Fluval A420 - G6 Phosphate Filter Cartridge is a replacement filter for the Fluval G6 Filter System. The Fluval G6 filter quickly removes up to 3900 milligrams of phosphate ion from marine or freshwater aquariums. The Fluval G6 replacement also manages algae growth, low oxygen, and unclean water. With this Fluval cartridge you will be able to manage low levels of phosphate.
Fluval A420 - G6 Phosphate Filter Cartridge specifications: