Abstract: This invention relates generally to the fluid of water tanks. More specifically this invention relates to water tank structure and quick change fittings for tanks which are adapted for the storage and dispensing of drinking water although any liquid could be used. The instant invention is a tank structure and fitting which permits the use of a pump or compressed gas to force the liquid out of the tank by the compression of a bladder within the tank which causes the liquid to be dispensed out of the tank when the bladder is collapsed. The fitting is sufficiently wide to facilitate the insertion, support and removal of the bladder or multiple bladders into and out of the tank. The fitting further permits the use of an inner flexible bag which is insertable into the bladder and allows the entire tank to be easily sterilized by simple removal and replacement of the inner bag without the necessity of removal of the bladder itself.