PURPOSE: To dissolve oxygen in water of a lake and a pond easily so as to purify water quality and improve an environment by means of a decorative fountain by injecting a fountain which has an irregular shape from the center of a cone shaped fountain directly upward and splashing away the fountain.

CONSTITUTION: An under water aeration pump floats in water by a float 5, and the pump is provided in such constitution that fountain formed in a thin layer is injected in cone shape above the surface of water by a complex type upper part impeller 20 installed on the upper end of the shaft 11 of a submergible motor 10 provided inside a cylindrical strainer 4 provided under the water surface. A pumping impeller 24 is installed on the bottom part of a motor shaft 11, and a water communicating hole 9 is opened on a motor shaft core to lead upward water which is sucked and discharged into a pump casing 7 installed on the bottom part of the strainer 4. A fountain nozzle plate 22 provided with an injection hole 21 is provided on the upper surface of an inside impeller 14 which rotates integratedly with a motor shaft and fountain injected in a cone shape is injected from a center directly upward.