Destratification
Destratification is a process in which the air or water is mixed in order to eliminate stratified layers of temperature, plant, or animal life. The first example of destratification was in 1919, in a small reservoir.[1]
A pond's condition deteriorates when the bottom environment cannot support animal life. The bottom is the area that runs out of oxygen first, it is where the most oxygen is used, and it is the farthest from the surface where it is replenished. Without oxygen a lake or pond's self-purification capability is not only reduced, it is reversed. The small animals, snails, worms, bacteria, etc., which help keep a pond clean cannot live, and the pond's nutrients are then recycled from the sediment. This forms a layer of muck at the bottom which serves as a fertilizer for weed and excessive algae growth. It can also cause large fish kills.