How to Maintain a Pond
Since Fountain Mountain focuses on large ponds rather than small, backyard ponds, we will concentrate here on the maintenance of a large pond only (at least a quarter of an acre in size). Your pond is an ecosystem, and though it might be a natural pond on your property, sometimes we can add human engineering to improve upon what’s already there.
The Banks
Planting along the banks can prevent erosion into the pond. Trees or grasses can prevent fertilizers from seeping into the water. You can also add embankments to deter erosion, or diversion ditches to move runoff water elsewhere. Be careful not to allow cattle or other large animals to graze upslope from the pond, or their wastes will get into the water.
Certain kinds of undesirable weeds tend to grow at the banks, as they need some soilA case in point is cattails. You have some weeds. Cattails and tall reeds develop rhizome mats that work like roots and they must be kept under control or they will be nearly impossible to get rid of. You cut down cattails, but the mats are still there, growing new cattails. Some tall reeds also have these mats, making them still another problem to get rid of as soon as possible.
Algae in the Pond 
Algae, in smaller amounts, is a good thing. Algae provide nutrients for fish, insects, birds, and it produces oxygen through photosynthesis. There are two main kinds we will discuss here, though really there are many, many more
There’s planktonic algae, which are microscopic plants which can bloom if they get excessive nutrients, like phosphorous and nitrogen. That is one reason why it is a good idea to have barriers at the banks of your pond so that these nutrients do not seep into the water from fertilizers that might be present on your property. Filamentous algae, also known as string algae, is also microscopic but gets “stingy” as it mats together and floats on the surface. Again, barriers can protect the pond from stormwater. If a pond gets too covered by string algae, fish kills are more likely and the rotten egg smell can develop as the pond becomes stagnant. In addition, more muck is going to develop at the bottom of the pond, increasing the likelihood of eventual dredging.
To prevent algae. you need to reduce nutrients seeping into the pond or lake. Be careful with fertilizers! You should not apply them on slopes that could leak down into the ponds. Adding pond dye can reduce algae growth by blocking sunlight.
Aerating Your Pond
An aerated pond is healthier
Aeration is important to prevent your pond from going into an anaerobic state. Proper aeration means that aerobic bacteria thrive, which create a harmless gas as they eat away at unwanted pond debris. You don’t get the rotten egg smell and a large buildup of bottom muck, which, over time, shallows out your pond. In addition, aeration mixes the warmer surface water with the colder, deeper water. As the water is churned, the temperature of the pond will be more consistent throughout. Algae will be decreased. Fish will thrive. Your pond will be healthy.
There are two main ways to aerate, on the surface or from the bottom. If your pond has an average depth of more than seven or eight feet, you should use a bottom aerator, but if it is shallower than that, a surface aerator works fine.
A surface aerator can be a fountain or a pure aerator. A pump is attached underneath a float which is connected by plumbing. If a nozzle is inserted on top of the float, then you have a fountain. The smaller the holes in the nozzle, the less aeration will occur, as less water will flow out the holes than if they are large. The simple, V-shaped design nozzle is used for aerating fountains, and typically it is one large hole that creates a V-shape only (this kind of fountain produces the most fountain aeration). Typically, the really, really tall fountains provide the least aeration, as to get the water up high and wide, small holes need to be used. When holes are very small, the nozzle creates a sprinkler effect, that is, a mist. When holes are large, the water droplets are large, too, and more water can be drawn from the pond and sent into the air around the float.
Pure surface aerators don’t include a nozzle at all. A surface aerator is merely a pump underneath a float, bubbling water at the surface. Water is drawn from fairly deep into the pond and brought to the surface. There is considerable agitation and mixing, reducing stratification in the pond. Yet these are most likely not to work well in ponds with an average depth of more than seven or eight feet. For these deeper ponds, it is best to use a bottom aerator.
A bottom aerator emits air bubbles from the bottom of the pond to the top, mixing and oxygenating the water that way. A diffuser or multiple diffusers are placed at the bottom of the pond (they typically sink to the bottom once dropped into the pond) and connected to tubing, which runs along the bottom of the pond to somewhere on the shoreline, where a compressor is placed. Many compressors are now much quieter than they used to be as technology is more advanced, plus they need to be housed to protect from the environment. You can buy such housing from manufacturers or design your own if you have the skill set. Or perhaps you have a boathouse or storage shed of some kind on your property already. Usually, the compressors do not need to be very big to provide sufficient air bubbles to aerate a large pond, thus reducing electrical bills for aeration and also reducing compressor noise. They are intended to be run 24 hours a day. 
What Kinds of Fish Can I Stock?
Certain kinds of fish will not take warm pond water. If you are planting trout, careful not to place them in a pond whose water temperature gets above 75 to 80 degrees. However, you can plant catfish, large-mouth bass, bluegills and sunfish in warm water. Usually there are enough nutrients in the water already so that you don’t have to feed them. Of course, you don’t want your pond to be overstocked, as there might not be enough oxygen in the water to keep them alive, and their fish wastes can make it too dirty.
Other Animals
Even in a large pond, you don’t normally want more than a pair of ducks, as they add their wastes to the water.
In summary, how to maintain a pond depends a lot on what you want your pond to do. Watch your banks to make sure there is not much erosion from them going into the water. Plant around your banks to decrease the possibility. If there are cattails and tall reeds along the banks take them out immediately as they can spread fast. Adding an aerator to your pond can greatly improve its overall health. If you have fish, do not overstock.