In accordance with the present invention an apparatus is provided to maintain separate upper and lower water levels in a bi-level tank. The purpose here is to maintain tank configurations for the water portion of an air/water pathway in the tank that will be followed by a buoyant power module during its electricity generating work cycle. During an operation, the module enters the tank and raises the lower water level. The apparatus is then expanded to displace a volume of water in the tank with a volume of air, which raises the upper water level. Subsequently, when the module leaves the tank, the upper water level is lowered. Further, the apparatus is collapsed to drop the lower water level back to its original level in the tank. Thus, the bi-level tank is reconfigured to receive another module, for another duty cycle.