A closed container of liquid is provided and operatively associated with a reservoir of water in a manner such that each time an intermittently actuated pump is actuated to pump water from the reservoir a predetermined amount of liquid is pumped from the container into the reservoir. The container has a bladder disposed therein and an outlet, separate from the bladder, for liquid to flow from within the container, exteriorly of the bladder, to the reservoir. The bladder includes an inlet and a water dispensing structure is operatively associated with the discharge line from the pump to dispense a predetermined amount of water from the discharge line of the pump into the bladder each time the pump is actuated, thereby causing an equal amount of liquid to be displaced from within the container exteriorly of the bladder into the reservoir.

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