In a fish tank, mechanical filter media is anything that traps waste, like a filter pad. Filter pads for fish tanks are measured in microns, which describes the pore size. So, a 50 micron filter pad has smaller pores and will trap more particulate, but will also clog faster than a 100 micron filter pad. Depending on pore size and need, aquarium filter pads should be changed about once a week. A poly filter pad is a special filter pad that traps waste and absorbs toxic metals. It will change color to let you know what is in the water.
An aquarium sponge filter is a special product for hospital, quarantine, and fry tanks. An air pump feeds the sponge, which houses beneficial bacteria.