A tank forcontaining a fluid, such as fuel, oil or gas within a chamber (7). The walls ofthe chamber are partially defined by a rigid structure havinganopening,- and partially defined by an elastomeric diaphragm (8) which is attached around its periphery to the rigid structure and covers the opening in the rigid structure. An attachment frame (9) engages the diaphragm and the rigid structure, and extends around the periphery ofthe diaphragm. The attachment frame has a first portion which engages the diaphragm, and a second portion which is positioned outside an outer edge of the diaphragm and attached to the rigid structure.