A submersible, water circulation system for enclosed tanks such as used by municipalities, fire districts, and industries. The system includes a driving unit having a shell extending along an axis with a pump supported within the shell. The shell has at least one inlet and at least one outlet and is positionable on the floor of the tank with the outlet facing upwardly. In operation, the pump continuously draws an incoming flow of water from outside of the driving unit adjacent the tank floor through the inlet of the driving unit and preferably drives all of the continuously incoming flow out through the upwardly facing outlet. The upwardly facing outlet is preferably a thin, elongated slot that creates a thin, substantially planar discharge of water therethrough that presents a very large surface area for its volume and induces water adjacent the shell to move upwardly with it.