The Hagen A425 is the correct chemical cartridge for the following systems:
The Fluval G3 Aquarium Chemical Cartridge - A425 fits the Fluval G3 Water Filtration System. The Fluval G3 filter keeps aquarium water clean and clear by capturing granulated chemical filter media down to 0.2mm. This Fluval chemical filter provides a healthy environment for the fish living in your aquarium tank. Fluval cartridges are easy to change and maintain without causing a mess.
Fluval G3 Aquarium Chemical Cartridge - A425 specifications: