Why Pond Aeration is Needed

Aeration Fights Stratification (Layering) of Ponds

Pond aeration is needed for fish health. If fish don’t get enough oxygen, they will die. Also, without aeration, even without fish, the pond’s overall health can decline, causing rotten egg smell, excessive algae, pond scum, and dirty water.

Fish Kill image and pond scum image above provided by Kasco

Stratification is the layering of water, often by temperature (ever notice the pond water is warmer at the top?) but also as a result of pond chemistry/structure and density. Here is a brief introduction to stratification terminology:

      Thermocline is the name of the separation between the pond layers. It is often the transition from the top, warm water and the cold bottom water.

      Euphotic Zone is the area of the water column that receives sunlight, or the top layer of the water.

      Epilimnion is the term for the warm water layer at the top of the pond during the summer months. 

Pond Turnover or simply Turnover is the process of a layered or stratified pond mixing once the density barrier is no longer present. This typically occurs in the fall but can also happen in the spring. It is often very abrupt and can be catastrophic to a pond environment. To learn more about your pond’s turnover cycle, specific information is included at end of this article*)

Prevent Turnover with Proper Aeration 

Aeration keeps the water mixed year-round, so the pond isn’t suddenly shocked by a sudden turnover mix. Aeration evens out the pond’s temperature, and oxygen is spread around so fish can thrive at various levels of depth. Necessary, natural bacteria thrive and bottom muck is reduced. The water becomes clearer, pond scum and invasive plants are reduced. Also, without aeration, ponds can become shallower over time from a buildup of muck caused by a lack of oxygen. Plant materials need oxygen to steadily decompose, and if new plant life grows at a rate faster than old plants decompose (due to a lack of oxygen), muck accumulates at the bottom of the pond. In addition, this muck sends up gas bubbles, which can create a very foul smell like rotten eggs. 



Surface Aeration for Ponds

The most common type of aeration is surface aeration. Surface aeration is also the pretty kind, that is, it is the use of pond fountains. Typically, the pump is attached to a float so that the nozzle rises and lowers with the water level. The pump agitates the water below, decreasing stratification. In addition, water is forced into the air so that the pond’s water is always being recirculated.

There are two common kinds of fountains used for surface aeration. Also, you can choose pure surface aeration with no display whatsoever

Display or Decorative Fountain Aerators 

These provide the versatile patterns and typically (not always) include multiple nozzles to change displays when you wish. The catch is that to provide so much display, nozzles can be intricate with many holes, and pump impellers are used. Fewer gallons per minutes of water are the result. Because of this, Kasco recommends using a 1HP pump for every ½ acre of surface area (a 100 x 200’ pond approximately). It should be noted that Kasco biologists and engineers make recommendations according to optimal pond health, and sometimes less HP works well enough. Also note that this ratio is for Kasco fountains. Some companies provide very tall or very wide displays, but to get that water so high the nozzles are restricting quite a bit of flow.

A Decorative Fountain

Aerating Fountains

Aerating Fountains provide only one kind of display, the V-pattern. The flow is not restricted by an intricate nozzle and the pump uses a propeller rather than an impeller. Kasco recommends 1 ½ HP of fountain for every surface acre of water (a 200’ x 220’ pond). Of course how much HP you need depends on the manufacturer of the unit you eventually purchase.

An Aerating Fountain with Light Kit

Surface Aerators without Display

A Surface Aerator (no display)

Surface aerators are only that, aerators. The pic above might appear to be decorative, and with lights added it would be. But it is 5HP! A propeller is used without a nozzle of any kind. Kasco recommends a 1HP to 1 acre ratio  (that is, 1 HP for every 200 x 220’ of pond). Of course, this varies with manufacturer and shape of pond.

Bottom Aeration

For very deep ponds (over 7 or 8’), bottom aeration is recommended. Typically, an air compressor is located at the shore and a hose carries the air to a diffuser unit at the bottom of the pond. It is very similar to the air bubbles rising from the bottom of an aquarium.  Currently, there are many fairly quiet air compressors available for sale. They need to be housed to protect from the elements, thus quieting them more.


Sizing Your Fountain or Aerator

Surface Aeration

Sizing Your Pond’s Surface

Remember, an acre is approximately 44,000 square feet. So a quarter of an acre is about 11,000 square feet, a half an acre approximately, 22,000 square feet. Of course, this is for rectangular shapes. (100 x110’ pond is a quarter of an acre approximately; a 100 x 220’ pond approximately ½ acre. We also can point to a calculator tool if you have a pond that is hard to measure because of its weird shape. 

Surface acreage can be calculated by this formula:

Length x width ÷ 43,560

Thus, 100 wide by 100 long= 10,000

10,000 ÷ 43,560 = .23 acres (about ¼ acre pond)

Sizing for Bottom Aeration (use of diffusers)

  1. To calculate your needs for bottom aeration, you need to know more than surface acre size. You need to know how many acre feet are in your pond. And we need to know your pond’s shape. Sorry it’s so complicated. You see, if you have a round pond that is 1 acre in size, a single diffuser system will work if it is 15’ deep, but if it’s only 10’ deep, you will likely need two.  Since sizing is so dependent on depth, we can’t do a lot of “rule of thumb” sizing. We go over details with our customers and get Kasco involved before we can make an official ruling on what is best for your particular pond.

But as a basic rule of thumb, acre feet is calculated like this:

One acre foot=1 acre of surface water that is 1 foot deep.

Acre foot= Surface acreage x  average pond depth.

Thus, a 10-foot deep pond an acre in size is 10 acre feet.



Combining the Two Kinds of Aeration

If you have a deep pond but still want to see a fountain, you can combine the two types of aerators. The fountain would not need to aerate as much and could focus on beauty. This focus allows for a greater range of price as well as dramatic viewpoint.

About Solar Aeration  

We get many calls about solar aeration. The goal is to eliminate the necessity of wire going to an electrical junction. While we are entirely sympathetic, these units are typically quite expensive, so don’t be alarmed by an approximately $8,000 price tag for a 1/2HP unit.

A Kasco Solar Aerator

Windmill Aeration

A Windmill Aerator

Wind units are pretty to look at, but installation time is lengthy, and you need to find the best point to install right off the bat, as moving windmills takes too much time. In addition, these do not turn when there is no wind, and when there is no wind rippling across the pond surface is the same time you need more aeration. 

*The information below is provided directly from Kasco Marine.

The Seasonal Progression of a Pond

We’ll start our seasonal progression in winter. The water temperature is fairly consistent throughout the entire water column. Warmer water is less dense than cold water and will float to the top. However, as water cools and gets gradually heavier, it is reversed at about 4 degrees Celsius and continues until the water is frozen (this is why ice floats). If you are in a northern climate that gets ice, there will be warmer water near the bottom of the pond that is warmed by the Earth and will not float to the top because the surface water under the ice is cold enough to be lighter.

As the ice melts in late winter or early spring, that 4 degree Celsius less dense water warms to above 4 degrees. When this happens it is denser or heavier than the warm bottom water. What then occurs is called Turnover. The heavier water is on top, but when the density barrier is eliminated the entire water column will be uniform density. This mixes up the entire water column and the pond will typically look dirty with suspended debris. Spring turnover is much less severe than fall turnover because there are not as much oxygen problems in spring as in late summer.

As spring progresses into summer, the top layer or epilimnion warms by the sun and increased air temperature. The warmer the weather and more intense the sun, the more distinct the temperature layering and greater density difference from top to bottom of the pond. The volume of the epilimnion can vary in pond to pond and from year to year, but is often very small in size compared to the colder, bottom water.

With fall comes cooler temperatures. The pond water will begin to cool throughout fall. The cooler temperatures will continue to cool the surface water until the water above the thermocline is the same temperature and density as the bottom water. Now you have a situation where turnover could occur. The quicker turnover occurs, the more severe the effects. A cool fall rain or cold front with lots of wind can quickly cool the surface water and cause a rapid turnover. Once the cooler surface water becomes the same density as the bottom water, the pond volume will mix and displace the water volumes. Again, the water column will be mixed and look dirty with suspended debris.

Fall Turnover is much more severe than Spring Turnover. During turnover, the closer the thermocline is to the surface, the worse the effects will be on the pond. If there is a large volume of water below the thermocline that has very little or no oxygen when turnover occurs, the low oxygen water gets mixed with the rest of the pond, thus decreasing the dissolved oxygen throughout the entire water column. Also, the chances of turnover are greater in fall due to common fall rains and cold fronts. This decrease in DO can be drastic enough to cause massive fish kills. Turnover will also mix bottom debris into the water column and can spur more bacteria activity, further lowering DO levels.

Once the water column is homogeneous again, the water will continue to cool at the surface as fall progresses into winter. Once the surface water hits 39F or 4C, it will float on the warmer, denser water until it cools enough to turn into a solid. At this time, your pond will freeze over and remain until the spring sun and warmer temperatures thaw the ice and start the process over again.

Again, by providing aeration to your pond, you are decreasing turnover by mixing the water year-round.

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