A reservoir system for an operating unit, such as a humidifier, includes a removable water tank configured to provide the operating unit with water. The removable water tank includes a primary reservoir for holding water, with an opening in the removable water tank fluidically connecting the primary reservoir with a secondary reservoir in the operating unit. A plunger is slidably disposed in the opening and movable between an open position in which the opening fluidically connects the primary reservoir to the secondary reservoir, and a closed position that seals the opening to fluidically disconnect the primary reservoir from the secondary reservoir. A float member in the secondary reservoir includes a buoyant main body and is configured to generate a force upon the plunger to urge the plunger into the open position according to the height of water in the secondary reservoir.