OXYGEN

Dissolving Oxygen into the Water

The only way oxygen can dissolve in water is through contact with the atmosphere. This is accomplished by diffusion between molecules of water in contact with the oxygen in the atmosphere. At the same time as oxygen is entering into the water, carbon dioxide is released or de-gassed into the atmosphere.

Some interesting deductions can be made from this fact.

a. All the water's molecules should be brought into contact with the atmosphere at some time during a complete circulation cycle.

b. The water should ideally be brought into contact with the atmosphere as much as possible (surface area and turnover rates) to assist with the diffusion of oxygen into the water and the release of unwanted gasses into the atmosphere.

If the water was drawn exclusively from the surface of the pond and returned to its surface, the oxygen levels in the lower levels of the pond will become critically low. Unless supplemental circulation within the pond is provided (like air stones) the water will take on a "dead" look. This statement is difficult to quantify. Experience will guide Koi enthusiasts to be able to identify water low in oxygen. This is similar to koi judges gaining experience and being able to appreciate different shades of white or what is called lustre when judging koi. In areas of low or no oxygen, anaerobic bacteria will flourish. The settled organics and solids will decompose and produce deadly Hydrogen Sulphide, gradually poisoning the system and fish.

Dissolved Oxygen

Dissolved oxygen is critical for the success of the pond and fish. High levels of dissolved oxygen will ensure good growth, health, lustre and colour.

The aim is to get and keep dissolved oxygen levels as close to saturation levels as possible.

Factors Affecting Dissolved Oxygen in Koi Ponds

Koi – the larger the individual koi and/or the higher the stocking densities the more oxygen is consumed and removed from the water.

Decomposing organic matter – this removes large amounts of oxygen and will compete with the koi for the amount of oxygen available in the water.

Altitude - there is less air pressure at higher altitudes and consequently less oxygen can be dissolved in the pond water. Ponds at altitudes of 1600m or 6000ft have about 20% less oxygen in the water because of less atmospheric pressure.

Temperature – temperature governs the metabolic rate of life in the pond and temperature also affects the amount of oxygen that can be dissolved into the pond water. The higher the water temperature, the less dissolved oxygen it can hold.

Dissolved Oxygen and Temperature

Temperature plays a major role in the dynamics of a Koi pond. Temperature governs the metabolic and growth rate of the fish and other life forms. Temperature governs the amount of oxygen that can be dissolved into the water.

The saturation level of oxygen in water decreases as temperature rises.

All forms of life in the pond, from the fish to the bacteria, are constantly removing oxygen from the water. As temperatures increase so does the oxygen consumption.

Organic matter such as dead algae and dead bacteria decompose and place an additional strain on the oxygen supply.

Algae use a vast amount of oxygen at night when the photosynthesis process is reversed and respiration takes place.

• See Turnover Rates and Flow Rates for suggestions on how to replace oxygen consumed in pond water.

The following chart demonstrates dissolved oxygen levels at different temperatures and different altitudes.

Dissolved Oxygen and Temperature Graph