The Fluval A418 is the correct replacement pre-filter for the following:
The Fluval A418 - G6 Fine Pre-Filter Cartridge is a replacement pre-filter for the Fluval G6 Filter System. The Fluval pre-filter captures excess debris in aquarium water for efficient mechanical filtration. The Fluval G6 replacement pre-filter can be used daily in marine aquariums or as a polishing cartridge for standard or freshwater aquariums. This Fluval cartridge is constructed with fine mesh that traps particles 75 microns or larger.
Fluval A418 - G6 Fine Pre-Filter Cartridge specifications: