Part 2 – Oxygen in a Koi Pond

Possibly the single most important element in pond water – and the most neglected aspect of Koi keeping. For life to exist in water there must be oxygen dissolved in it. Whilst there are around 210 000 part per million of oxygen in the atmosphere there are only around 8 parts per million in water – in a well aerated pond. A small increase in oxygen levels in pond water makes an enormous difference to the activity of the fish and other life forms. The fish, the bio-filter, organic decomposition etc. all extract oxygen from the system. Oxygen can only be introduced into the pond via contact with the atmosphere through design considerations such as streams, waterfalls and apparatus such as venturis and air blowers. High turnover rates ensure a high oxygen level. Photosynthesis by algae during the day introduces additional levels of oxygen, but removes it at night. Air under pressure through venturis 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 temperature. There can be significant oxygen variations in ponds with poor circulation or in ponds which do not move the water away from the bottom of the pond i.e. the point furthermost from the atmosphere. There is less oxygen in pond water at higher altitudes – about 18 to 20% less than at the coast. There is continual competition in the pond for the limited amount of oxygen available at any given time. The fish, the plants, the micro organisms all need oxygen rich 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 sunlight may rocket oxygen levels to saturation 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 will the stocking densities of fish. The faster the turnover 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 breathing all the time.

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

A shortage of oxygen will be noticed by observing the Koi collection first thing in the morning. If the Koi are moving lethargically and hovering near the surface you may well have an oxygen deficiency. Oxygen shortages have been measured in ponds with fancy pumps and filters – but with a lack of exposing the water molecules to the atmosphere. A shortage of oxygen can also be observed by watching the breathing of the fish. If the fish are breathing heavily or “piping” they could be 1. Stressed or 2. Have a gill problem or 3. The pond water could be low in oxygen or 4. The fish could be exhausted for some reason (and what have you been doing during the night to exhaust yourself, my little Cynthia Sanke?).

Keep in mind that the atmosphere is composed of only about 20% oxygen. Nitrogen makes up the bulk of the rest of the atmosphere. When using venturis i.e. pumping air into ponds under pressure, you are not only pumping in oxygen but nitrogen and other gases found in the atmosphere. High nitrogen levels will affect the behavior of the your Koi. They will become lethargic and there will be some loss of appetite. You may also notice a discoloration of the white on the Koi.