Storage tank assemblies and methods for water on water reverse osmosis systems. A bladder type storage tank is fabricated by forming first and second shells, which when joined together at a center region of each shell, form a tank enclosure, the first shell having first and second openings therein away from the center region of the respective shell, coupling a bladder to the first opening in the first shell so that the bladder will be within the tank enclosure when the first and second shells are joined together at a center region of each shell, temporarily holding the bladder in a collapsed state away from the center region of the first shell with one or more strings and passing the strings through the second opening, welding the first and second shells together at the center region of each shell to form the tank enclosure, and withdrawing the strings to release the bladder.