Some recommended ways to aerate include:
Aerators add oxygen to the pond using a special pump. The warmer the water, the more oxygen is needed.
Spitters are small statues that spit or pour a stream of water into the pond. They are usually placed on or near the edge of the pond.
Nozzle kits offer you a variety of water displays which you can adjust to varying heights depending on the power of your pump.
Waterfalls not only help to circulate the water, they help oxygenate the water as well.