Aeration, very simply, is the transfer of oxygen from air into water.  All healthy bodies of water need oxygen for the normal biological cycle to be completed.  In large bodies of water this occurs naturally as a result of wave action.  In smaller lakes and ponds, it is easy for biological factors to become unbalanced, causing the oxygen level to drop, making it necessary for mechanical aeration. 
 
Insufficient aeration and low oxygen level can cause fish kills, obnoxious smells, and unsightly water.  Without sufficient circulation, a pond becomes stratified with warmer water on the surface and colder, oxygen-depleted water on the bottom.  Lack of oxygen increases anaerobic (without oxygen) decomposition of the sludge at the bottom.  That increases nutrients in the pond, which, in turn, increases algae blooms.
 
An un-circulated or un-aerated pond or lake develops a thermocline.  This is a stratification of the water by temperature.  The warmer water is at the surface in contact with the air and the cooler water drops to the bottom where it becomes oxygen starved.  Proper circulation and aeration will increase the amount of oxygen at all depths, and break down the thermocline.  Cool water holds more oxygen than warmer water does, so mixing the cooler water from the bottom with the warmer surface water enables the entire body of water to hold more oxygen as well.  Aerating a pond provides a healthier habitat for fish, reduces algae blooms, improves water clarity, and rids the pond of bad smells.  In short, aeration aids the normal biological life cycles in a pond so they work the way they should.  Aeration also improves the effectiveness of bacterial products that reduce nutrients and improve water clarity. 
 
The amount of aeration required for a pond or lake depends on its surface acres, its depth, whether it collects run-off from fertilized areas, and its water quality.  One (1) horsepower of surface aeration per surface acre should be adequate aeration for a typical pond.  Multiple smaller units can be more effective than one large unit if the pond is shallow or has an irregular shape.  The smaller the pond or lake, the more aeration it needs per acre. 

ACI offers three types of mechanical means to add oxygen to a lake or pond: A surface aerator (Display Aerator or Torrent), a bottom diffuser (Lake Bed Aerator), or a Bottom Circulator.