What Kind of Aeration is Best for My Pond?
Aeration Reduces Stratification (Layering) of Ponds
Pond aeration is needed for fish health. One of the worst things that can happen to a pond is fish kill (see photo below). But even if you don’t have fish, without aeration, ponds create stratified layers in the water, and the bottom layer is depleted of oxygen. Also, the pond surface can become quite stagnant and full of muck. The accumulation of muck will cause your pond to become shallower over time, perhaps necessitating dredging. Plants, including algae, in the pond need oxygen in order to decompose, and if they don’t get enough of it from proper aeration, old plants will decompose faster than new ones, and the muck sends up gas bubbles which can create an awful odor.
Common Ways to Aerate
Basically, there are two different ways to aerate, from the top of the water and from the bottom.
Surface Aeration
The most common type of aeration is surface aeration. A pump is attached to a float and the unit rises and lowers with the water level. The pump agitates the water below, decreasing stratification by churning the water. On top of that, water is forced into the air so that the pond’s water is always being recirculated. But you should know that surface aeration only works to a depth of 7 or 8 feet. The pump won’t pull on the water that far below the surface.
When using surface aeration, you have three choices. The first one is the decorative fountain, as a nozzle is attached to the top of the float to create a desired flow pattern. The holes which create the pattern also have the effect of restricting water flow, so pure decorative fountains do not aerate as well as the other two kinds of surface aerators.
The aerating fountain is also a fountain display but only offers a single pattern, the V-shape. Water escapes the top of the pump through one very large hole, thus not restricting water flow as much. 
The pure surface aerator is comprised of only the float and pump. There is no restriction at all on the water output, and it merely bubbles or looks as though it’s boiling at the surface.
Bottom Aeration Using Diffusers
For very deep ponds (over 8’), bottom aeration is recommended. An air compressor is located at the shore and a hose carries the air to a diffuser unit at the bottom of the pond. Though compressors used to be rather loud, nowadays they are much quieter. They are also very energy-efficient, and a great deal of aeration can be done with a quarter horsepower compressor. They need to be housed to protect from the elements, thus quieting them more. If your pond is shaped conventionally, as a rectangle or oval, then it’s often the case you will only need a single diffuser at the bottom of the pond. If it has islands or unusual shapes, there’s a good chance you will need to place more than one diffuser at the bottom. 
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
The goal is to eliminate the necessity of cords going to an electrical junction and to sidestep electricity bills. These units can be quite expensive, but they are built to be used with both surface and bottom aeration. Unless you have battery backup, these units will only run while the sun is out. And even when the sun is out, how sunny it is and direction of your solar panels will change the amounts of solar energy, so it’s a good idea to use a pump with variable speed.
Windmill Aeration
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 aeration is most necessary, especially during the summer.