Mechanical Filter Sponge for Zoo Med's 501 External Filter. Zoo Med's Mechanical Filter Sponge is the first line of defense in removing dirt and floating debris from your terrarium or aquarium. Although filters are used to keep your water clean and safe, Zoo Med also recommends weekly water changes of at lest 20%. When adding tap water (after a water change) make sure you ad Zoo Med's Reptisafe to remove chorine and chloramines.