This invention relates to a diffuser for an aeration system, including a base, a diaphragm and a valve member attached to the diaphragm. The base is provided with a valve seat for engaging with the valve member to close the inlet of the aerating gas. Furthermore, the diaphragm includes a central portion, a peripheral portion attached to the periphery of the base, and a surrounding web segment interposed therebetween. The surrounding web segment includes a plurality of fibrous filaments which are arranged to form a textured structure with a plurality of pores of a dimension such that the aerating gas, when introduced, is permitted to be bubbled through the plurality of pores, so as to form fine and small bubbles in the water.