Oxygen is possibly the most important element in pond water - and the most neglected aspect of koi keeping.  For any life to exist in water, there must be oxygen dissolved in it.  The fish, bio-filter, organic decomposition, etc. all extract oxygen from the system.  Oxygen is introduced into the water by gaseous exchange between the atmosphere and water molecules.  Oxygen is introduced into the pond via contact with the atmosphere through design considerations such as streams, waterfalls and apparatuses such as venturies and air blowers.  High turnover rates ensure a high oxygen level.  Photosynthesis by algae during the day introduces addition levels of oxygen, but removes it at night.  Air under pressure through venturies can cause gas imbalances in the water and need careful consideration in their placement in the pond.

  Oxygen levels can fluctuate dramatically during the day and night as well as during periods of high and low temperatures.  There can be significant oxygen variations in ponds with poor circulation or in ponds that do not move water away from the bottom of the pond.  i.e. the point farthest from the atmosphere. Temperature also has an effect on dissolved oxygen, the approximate saturation level at 50° F. is 11.5 mg/l, at 70° F., 9 mg/l, and at 90° F., 7.5 mg/l.  There is less oxygen in ponds located at higher altitudes - about 18 - 20% less than at the coast at sea level.  Impurities such as salt will further decrease dissolved oxygen levels.

  There is continual competition in the pond for the limited amount of oxygen available at any given time.  The fish, plants and micro organisms all need oxygen enriched water ALL the time.  Algae and submerged plants have a dramatic influence on oxygen levels in a pond during a 24 hour day/night cycle.  The photosynthesis process during the sunlight may rocket oxygen levels to saturated point and beyond.  However, a dramatic plunge in oxygen with the reversal of the photosynthesis process at night can spell disaster, even to the point of fish suffocating in ponds at dawn.

  It has been found that if the oxygen levels are 25% below optimum levels first thing in the morning, growth rates are reduced.  The turn over rate will have a direct bearing on oxygen levels as well as the stocking densities of fish.  The faster the turn over rate the more water will come into contact with the atmosphere and the more gaseous exchange will take place.  The more fish in the pond, the less oxygen in the water, as they are all breathing all the time.

Higher temperatures in summer mean there is less oxygen that can be dissolved into the water.  Higher temperatures means a faster metabolism, which in turn means the less oxygen available is extracted faster and is needed in more quantities by the fish and other life forms than in colder temperatures.