Second Nature Products ASN65508 Wet/Dry Filter Trickle Sponge Replacement for Wet-Dry Power Filters 3000 and 4000 Wet/dry filter trickle open cell sponge replacement It offers extraordinary surface area Allows huge amounts of oxygen laden water to trickle through and stimulate a massive growth of aerobic bacteria This bacteria is a great natural purifier of aquarium water This replacement trickle sponge is for wet-dry power filters 3000 and 4000