Fluval C4 Bio-Screen Pad provides massive biological surface area for friendly bacteria to colonize, plus it blocks debris and evenly distributes water flow before it drips into the Power Filter's biological trickle chamber.
Key Features :

Provides massive biological surface area for friendly bacteria to colonize
Blocks debris and evenly distributes water flow before it drips into the Power Filter's biological trickle chamber
For Fluval C4 Power Filter
3-pack

Other sizes available :

14020 Fluval C2 Bio-Screen Pad, for Fluval C2 Power Filter
14021 Fluval C3 Bio-Screen Pad, for Fluval C3 Power Filter,