Forza Replacement filters fit the Rena OEM Forza 15-25 gal Forza Power Filter. Slide into the slot on the filter for easy filter change. The filters feature all three recommended filter types Carbon (chemical), White (particulate/biological) and a plastic grill which removes larger particles. Depending on the aquarium load change the filter every 1-2 months. They can also be rinsed out in water from the aquarium in between changes. See filter directions for further details.