A non-metallic, diaphragm-type tank assembly for use with a pressurized water system is disclosed. The tank assembly comprising a non-metallic outer body; a non-metallic inner shell assembly, including an upper portion and a lower portion, that is contained by the non-metallic outer body; and a diaphragm that is structured and arranged about the upper and lower portions of the inner shell assembly to separate said inner shell assembly into a water portion and a pressurized gas portion. Preferably, the diaphragm comprises a resilient, non-porous material and includes a bead portion comprising an annular ring that is convex on an inner side and concave on an outer side at its outer periphery. More preferably, the bead portion is removably secured to the overlapable end portion of the lower portion of the inner shell assembly by a clamping system that comprises an inner clamp hoop and an outer band. Specifically, the outer band can be mechanically crimped to compress and secure the second overlapable end portion and the bead portion of the diaphragm between the inner clamp hoop and the outer band to provide a watertight seal for the water portion.