Low levels of dissolved oxygen in ponds are likely the most common water quality problem in fish ponds. Aeration is often necessary to prevent fish from dying during low dissolved oxygen periods. Aeration of a pond is accomplished through the dropping of water at a waterfall, using a venturi fountain head on a small submersible pump or an air compressor aeration system.
Water can hold only a limited amount of dissolved oxygen. It increases as atmospheric pressure goes up and as temperature and salinity go down. A couple of way dissolved oxygen is added to water is by atmospheric diffusion at the surface and by photosynthesis during daylight hours. However, in a closed pond ecosystem, the majority of dissolved oxygen is supplied by the water fall, stream and/or fountains. Dissolved oxygen levels of 3-5 ppm will adequately support most pond fish.
A number of conditions either alone or in combination may develop which result in oxygen depletion. Hot, cloudy, still weather is common during the summer months. Warmer water holds less dissolved oxygen than cooler water. Although cloud cover may make the air cooler and hopes of cooler water, cloud cover also limits light and slows photosynthesis. On calm days, the surface of the pond will becalm calm as well unless there is a stream or water fall with enough force to ripple the surface. Another aid to the surface diffusion is the use of a fountain that will cause surface rippling. Additionally, pond fish are more active during the summer months, when water is warmer, and thus their oxygen requirement is at its greatest.
The most catastrophic low dissolved oxygen event is called an inversion or more commonly pond turnover. This problem is especially acute in deeper ponds. As the upper 4 feet or so warms, it becomes lighter and does not mix with the cooler, deeper water. Eventually, the bottom water becomes un-oxygenated as the dissolved oxygen is consumed in various biological and chemical processes that take place there. A sudden rain or even a cold front, with strong wind can cool the upper water to the extent that is becomes heavier and sinks. The deep, anoxic water is forced to rise and combine with the surface water causing an oxygen reduction and either a partial or complete fish kill.
Another oxygen depletion situation occurs sometimes in ponds having high densities of fish that are being fed. In such instances, more oxygen than is available in the pond water may be required by the large fish crop and in the waste decomposition process.
Artificial aeration in any of these low dissolved oxygen situations can prevent fish from dying. Investment in aeration of some kind is the best insurance possible for a fish pond.