Abstract: An electrolytic chlorine gas generator is coupled to the return pipe of a pool water circulating system for introducing a chlorine-hydrogen gas mixture into the pool water. A gas/liquid delivery tube connecting the generator with the return pipe is sized to have an internal volume just sufficient to hold a quantity of gas which, when under pressure, has a volume substantially equivalent to the sum of the increased internal volume of the generator when subjected to return water pressure plus the decrease in the volume of the gas trapped in the vessel due to compression when subjected to return water pressure. Depletion of electrolyte in the generator increases the volume in the generator and allows pool water to enter the generator through the gas/liquid delivery tube to replace spent electrolyte automatically when the generator is pressurized.