The O of H2O or Pond Aeration Basics
You most likely know that your pond needs oxygen in the water but you may not know how much and how to get it there.
A still pond is only able to exchange gases at the water surface. A still pond will support only a very small fish population.
A pond not only needs to absorb oxygen from the air but it also needs to release carbon dioxide and maybe hydrogen sulfide among other gases. Hydrogen sulfide is produced by anaerobic (without oxygen) bacteria breaking down organic matter in the bottom of the pond. The pond bottom has less oxygen than the upper layers of the pond especially if you have stone or gravel in the bottom of your pond. Anaerobic bacteria work in the absence of oxygen and cause a septic condition in the bottom of the pond.
When you spray water through a fountain or when you run water through a stream or over a waterfall you are increasing this surface that is exposed to the atmosphere many times. Harmful gasses can be released to the atmosphere and oxygen absorbed very easily.
You want to circulate the water in the deep part of the pond in order to help with the break down of organic compounds therefore providing better water quality. During warm weather draw water from the bottom of the pond to expose it to the atmosphere. When you expose the bottom of the pond to oxygen rich water you will have aerobic (uses oxygen) bacteria breaking down the organics. This will reduce foul odors in the pond, which are caused by anaerobic bacteria action. 
Since oxygen is absorbed only through the surface of water it is possible to have too many plants in a pond. If the entire surface is covered with water lilies or other surface plants very little gas exchange can take place unless there is a large fountain or waterfall.
You may think that adding more underwater plants will add oxygen to the water. While underwater plants will help add oxygen to the pond during the daylight it will use up the available oxygen at night the same as other life in the pond. As such it is possible to have too much anacharis or other underwater plants in a pond. Underwater plants are important to the pond but their function is to use up nutrients to starve out the algae.
There is a limit to the number of fish that you can keep in a pond. When you exceed this number you start having problems with water quality and fish health.
The hotter the water the less oxygen it can hold. Therefore cold water holds much more oxygen and coupled with the slow respiration of fish in the wintertime you should not need additional aeration in the pond.
If your fish spend a lot of time at the surface sometimes gulping air or you have noxious odors coming from your pond then you most likely do not have enough circulation. What steps can you take to insure an ample amount of oxygen in the water? We have already mentioned that it may be necessary to remove plants and fish when these become excessive. If your waterfall or fountain is not adding enough circulation then you may want to consider a larger pump. You may also want to consider adding an additional fountain or waterfall. By simply adding an aeration pump you can greatly increase the gas exchange with the water.
In very hot weather a pump failure can lead to a fish kill very quickly. By having a second pump running or an aeration pump you greatly decrease the chance of having a major loss of fish. Even if you don't keep it in operation, having a backup pump available in case your main pump fails can save your fish. If your pump fails in hot weather you may only have a few hours if your pond is heavily stocked with fish.