The aeration system is for aeration and/or mixing of water, which system has at least one aeration unit that has a pump/propeller inside a feed pipe. The feed pipe, to which the water to be aerated, is sucked from beneath. The aeration system has the feed pipe expanding in the upper part of the aeration unit to a conical space working as a nozzle, via which the water continues its way to at least one annular opening.