Poret® foam is a water-proof polyether foam with a particular open-cell structure. The reticulation process produces well defined open pores. This foam is not the same as foam commonly used for mattress, isolation or packing, since these foams are closed-cell foams. Many sponge filters available on the market are made of isolation or packing cell foam because it is much cheaper. However, these cheaper foams have closed air bubbles that are inaccessible to filter bacteria or water and consequently, they do not work very well. Polyether is resistant to hydrolysis and has outstanding aging, temperature (up to 250°F/ 90°C), and chemical stability when compared to other foams. The physical properties of Poret® such as compression and tear strength, and elongation are greater than conventional polyurethane foams. Poret® is also highly ozone and oxygen resistant, which is crucial for marine aquarium applications. Poret® foam does not contain toxic chemicals such as plasticizers. Poret® is specifically manufactured for aquarium and pond use and has a much larger inner surface than other filter materials (about 1800 sq inch per quart or liter of volume), which permits more rapid and superior growth of bacterial cultures. The foaming process is fluorocarbon-free and the foam can be disposed of safely by incineration. The size of pores is calculated in pores per inch (PPI) and the foam is made in pore sizes from 10-45 PPI.