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.
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.
To find products to help treat ammonia buildup, check out Pond Maintenance > Water Treatments > Low Oxygen Levels