A filter assembly for use in filtering and treating contaminated water in an aquarium tank. A filter bag (10) is used for receiving a continuous supply of contaminated water (14) under a predetermined pressure. The walls (12) of the filter bag are made at least partially of a porous filter membrane formed of a matrix of pressure displaceable fibrous material. As the filter clogs with contaminants, bacterial growth and accompanying by-products, the fibrous material is displaced to increase the size of the interstices of the matrix to permit water to continue to flow through the interstices and continue filtering.