Locking apparatus is mounted to a wall or inside a bag-supporting container. The fluid-storage bag has a tubular fitting with axially spaced flanges forming an annular locking area. The locking apparatus comprises a diaphragm mounted over a port formed in the wall. The diaphragm has a opening formed therein which has a size and polygonal profile which matches the polygonal perimeter of the fitting's locking area. Cuts extending radially from the vertices of the polygonal opening form radial flaps. The outermost flange is larger than the diaphragm opening and when forcibly pushed through the diaphragm opening it deflects the flaps. Once the outermost flange passes axially through the flaps, the flaps snap back flexibly and engage the fittings locking area, resisting fitting removal. The complementary polygonal tubular perimeter and diaphragm opening resist rotation aiding in installation of threaded accessories. A removable plug is provided for installation to port outside of the wall which, when installed, seals any fluid within the walled enclosure.