As a stream is constantly running, the water in that stream, as it churns over the rocks, is regularly in contact with the surface where it can receive oxygen from the air however, in the case of a pond, the water is still which means the water at the bottom of that pond never reaches the surface and so has a lack of oxygen. At first the water at the bottom is only deprived of oxygen but as time goes by a larger portion of the pond becomes starved of oxygen and fish that may be in that pond will start to die. Also the lack of oxygen in the water will encourage the growth of algae which can cause the pond to start to become unsightly and will eventually start to give off a foul smelling odour. To ensure that your pond does not degenerate in this manner, you will have to ensure that the water receives adequate oxygen by aerating it. A popular pond aerator is a fountain. A fountain, as it circulates the water in the pond, will ensure that all the water in that pond is adequately aerated but is only effective for shallower ponds, not for ponds which are perhaps deeper than about 5 or 6 feet.
For deeper ponds you will need an aeration system that is specifically designed for deeper waters and these consist of 3 components, a diffuser, a tube and a compressor. The compressor pushes air through the tube to the diffuser which, in order to ensure that all the water in the pond is adequately aerated; is located at the bottom of the pond. Obviously as the air rises through the water, it oxygenates that water and also causes the water to circulate. As the compressor has to push air to the diffuser, obviously it needs an electric motor and as batteries are perhaps a little limited in their use, usually these motors are connected to the mains electricity system. Sometimes though, due to the location of the pond or just because no addition is wanted to the mains electricity bill, other sources of power are provided in the aeration systems. If you have a shallow pond, you may want to get a solar powered system which are effective and can work even if they do not see sunlight for as many as 3 days. For both shallow water and deeper ponds, aeration systems that are powered by windmills are available. As the sails of the windmill rotate, they provide power to the motor and so no addition to the electricity bill is necessary.
Obviously as fountains can add beauty to any pond, they are very popular but it must be stressed that although a fountain can also act as an aeration system as well as an added attraction, they are only effective as aerators for shallow ponds and not for deeper ones however, a deep pond can have a fountain, provided that it also has an additional aeration system, one which is designed for use with deeper waters.