An aerator for injecting air into water includes a hollow tube having an inlet end and an outlet end, a hollow dish-shaped element including an upper conical portion and a lower conical portion, blades and a rotating element. The upper conical portion is connected the outlet end. The lower conical portion is located under the upper conical portion. The lower conical portion is spaced from the upper conical portion by a first distance. Each of the blades is connected to the upper conical portion and the lower conical portion, and the blades are arranged in a spiral shape. The rotating element rotates the hollow tube, the hollow dish-shaped element and the blades. The blades strike the water to suck the air through an air intake hole of the inlet end into the hollow dish-shaped element, thereby injecting the air into the water under negative pressure.
