The EHEIM Fine Filter Pad Classic Series is a white, phenol-free foam pad designed for use in the Classic line of EHEIM external canister filters. It captures very fine particles and suspended debris while providing a porous surface on which beneficial bacteria can establish and contribute to biological filtration. The pad is reusable simply rinse gently when dirty (rather than replacing immediately) to preserve the bacterial colony but should be replaced when it becomes clogged or damaged to maintain optimal flow through your filter. Compatible with both freshwater and marine setups.