Abstract: A wave-making water return device for use in an aquarium, which is constructed to have an inlet to receive water, an outlet to discharge the water into the aquarium, a fluid connection between the inlet and outlet, and an actuator to rotate the outlet so as to make waves of the water in the aquarium. The outlet is rotatable with respect to the inlet around an axis concentric with the inlet. The fluid connection includes an inner pipe coupled with the inlet and rotatably supporting an outer pipe fixed with the outlet. The outer pipe is rotated back and forth around the inner pipe by the actuator. The actuator includes a motor having a motor shaft, a first rigid arm rigidly coupled with the motor shaft, and a second rigid rod pivotally coupled with the first rigid arm and the outlet.