The Hagen A1377 is the correct replacement media for the following:
The Fluval A1377 Spec Carbon Filter Cartridge 3-Pack is a three pack of replacement media for the Fluval Spec. This Fluval carbon filter removes heavy metals, odors, and discoloration in your aquarium and also absorbs impurities to keep your tank clean and clear. This aquarium carbon filter provides Stage 2 activated carbon filtration.
Fluval A1377 Spec Carbon Filter Cartridge 3-Pack specifications: