Aquarium filter pads are thinner than foam blocks; use them to make layers around other media for extra protection against clogging. They can contain additional filter media like activated carbon; and some are meant to be cut into whatever size or shape you need. Then there are those intended for specific filters that pop right into place for easy replacement. Aquarium filter pads come in various pore sizes (coarse-fine; pores are measured in microns) and have various attributes.
