This filter is for salt-water use only. A sponge filter looks like a tube with a sponge like material inside. As water flows through, bacteria will colonize the porous foam and establish a biological filtration. These sponges also serve as a mechanical filter, removing larger particles from the water. The advanced versions use two sponges, making it easier to preserve bacteria colonies by replacing the sponges at different times.
Water is pumped upward through the gravel by air bubbles, water stream, or a combination of both.
This slow flow of water and oxygen allows the bacteria to colonize the gravel. The UGF is an aid for biological filtration. It does not remove larger waste particles. It has to be well maintained, especially through vacuuming of the gravel. UGF s are inexpensive, but have a tendency to clog up.