Fluval Aquarium Filter Foam Blocks provide mechanical straining of particulate waste. It prevents particulate waste from prematurely clogging other filter media in the module stack. Although the primary purpose of Fluval Foam is mechanical filtration, biological activity will eventually occur within its pore structure. These Fluval Aquarium Filter Foam Blocks also enhance biological filtration and are suitable for the Fluval 204/205/206 and Fluval 304/305/306 Aquarium Filters.