You will receive several view points on aeration in every situation. Info that would be helpful for figuring things out will be shape of pond, depths, and fertility of the bottom sediment. Diffused air from an electric motor(piston, or rotary vane) will be your best option for deeper water. Windmills are for specific applications(no power available,primary goal of keeping fish alive over winter, and places exposed to constant wind). Fountains are only efficient in shallow areas. You can save a bundle by doing it yourself from some of the catalog companies mentioned on this site. Do your research on some of the other posts on air, and remember you can't aerate too much during summer, but be careful during the winter. If you can get someone with knowledge to come out it is money well spent.
Robert B