Summer is one of the best times of the year for pond owners, as the warm weather allows us to sit outside and really enjoy our ponds.

Oxygen is essential for the well-being of fish and other aquatic life, being required for aerobic respiration, which involves the breakdown of glucose to release energy. Whilst oxygen is abundant in air, there is only about 20 times less of it in water, making respiration all the more difficult for aquatic life. The oxygen content of the water is therefore something that aquarium and pond owners need to consider when managing water quality.


A number of units and measures are used when describing oxygen levels, and it is useful to have some idea of what they mean:

ppm or mg/l = parts per million or milligrams per litre

There are one million mg in a litre of water, so one part per million is equal to one milligram per litre. An oxygen level might be described as 10mg/l, meaning that there are 10 milligrams of oxygen for every litre of water ( one litre = 1,000,000 milligrams). This might also be written as 10ppm.

% saturation = the amount of oxygen held in the water,
as a comparison with the maximum amount that could be held in it

For instance, at 10C freshwater can hold a maximum of 11.28mg/1 oxygen. If the actual amount in the water at this temperature is only 9mg/l, then the saturation value is 80%. If it were 11.28mg/l, the saturation value would be 100%. It is easier for fish to remove oxygen from well-saturated water.

Measuring oxygen levels is simple, using TetraTest Oxygen test kits, which contain all the information you need to ascertain how well oxygenated your aquarium or pond water is.


The oxygen requirements of fish vary considerably, depending on different factors. These include the size of the fish, species, the temperature of the water, quality of the water, the amount of food the fish is consuming, how active the fish is, and so on.

The oxygen demand of a fish can easily double a few hours after feeding, as it uses energy for digesting and processing its meal. Temperature can have an even more dramatic effect, as numerous studies have shown. For example, a 23g Channel Catfish needs approximately 2.53mg of oxygen per hour at 1.7C, whilst at 18C it needs nearer 8.46mg per hour, more than a three-fold increase (source: Lawson, 1995). In some cases this effect may be even more profound, for example a Goldfish may have an oxygen demand 6 times higher in the summer, than in winter. In most cases, coldwater and pond fish need an oxygen level of at least 6mg/l, whilst tropical fish need 5mg/l. However, levels should ideally be maintained at 7mg/l. Although small fish require more oxygen per unit of bodyweight, larger fish require greater overall quantities. Therefore, they are usually the first to suffer when levels fall below optimum.


Many different factors can either directly or indirectly affect the oxygen level of the water:

Temperature: This has perhaps the biggest effect on oxygen levels. The warmer the water is, the less oxygen it is able to hold. Combined with the increased oxygen demand of fish at higher temperatures; warm water can quite easily become oxygen deficient. The following table lists some oxygen concentrations at different temperatures:

Temperature (oC)                                    Maximum Oxygen Content (mg/litre)

    10                                                              11.28
    15                                                              10.07
    20                                                              9.08
    25                                                              8.24

At very low temperatures, or if the temperature falls very rapidly, the ability of the fish to remove oxygen from the water can be impaired, making oxygen deficiency a possibility. However, in most cases fish are able to adjust to falling temperatures in time to avoid harm.

Atmospheric pressure: At higher pressures, water is also able to hold higher quantities of oxygen. This effect can be important in the summer, particularly in ponds, when low pressure conditions (such as those experienced in storms) are combined with warm weather. Such conditions can often result in the loss of more oxygen sensitive species such as Orfe, if adequate aeration is not provided.

Aquatic respiration: The abundance of aquatic life, including plants and microbes as well as fish, will determine the total oxygen demand of the pond or aquarium. The higher it is, the greater the demand will be. In ponds and tanks that are rich in life and well stocked, oxygen levels are more likely to become critical.

Salinity: The higher the salt content of the water, the less oxygen it can hold. Therefore, marine aquaria require more efficient aeration and are more vulnerable to oxygen depletion. In freshwater aquariums and ponds, salinity is not a concern, even if therapeutic doses of salt are being used.

Organic matter: Any organic matter that is present in the pond or aquarium will be subject to breakdown by various microbes and invertebrates. This process uses up oxygen, and so excessive levels of sediment can lead to very low oxygen concentratIons. The same thing can happen in the filter if it is not kept reasonably clean.

Treatments: Certain strong medications can reduce the oxygen concentration of the water. If you are unsure about treating, always use a product you know to be safe, such as TetraPond MediFin or the TetraMedica Range, and read the instructions carefully.

Algae: Excessive algae or oxygenating plants can cause the oxygen level to fluctuate dangerously. During the day they photosynthesise and produce oxygen, but at night this stops, yet they continues to respire, using up oxygen. This can lead to dangerously low oxygen levels in the early hours of the morning.

Green water can be particularly efficient at removing oxygen from the water. A good rule of thumb is that if you can't see beyond about 30cm into the water, oxygen depletion at night is likely.


To ensure that oxygen levels are maintained, follow a few basic guidelines:


Try to have at least one fountain or waterfall in the pond, and keep it on 24hrs a day, especially in the summer. The more surface agitation there is the more oxygen can diffuse into the water.
Stick to sensible fish stocking levels; keep oxygenators under control.
Prevent excessive algal growth through the use of a TetraPond Ultraviolet Light, AlgoRem or AlgoFin.
Keep filters reasonably clean, regularly remove any trapped sediment.
Keep the base of the pond free from sediment, through the regular use of a pond vacuum, or use a TetraPond OFX pump connected to a filter system.
Use a TetraTest Oxygen kit to keep an eye on oxygen levels.
The best time to test the water is first thing in the morning, when levels are likely to be at their lowest.


Make sure that filters are kept clean and working efficiently, those such as the Tetratec IN filter are designed to aerate the water as well. An extra AP air pump can be added to guarantee oxygen levels.
Stock the tank sensibly.
Control algae through the use of Tetra EasyBalance and Tetra NitrateMinus.
Use a Tetratec HydroClean gravel siphon to keep the substrate free from excessive dirt.
Use a TetraTest Oxygen kit to monitor levels.


If low oxygen conditions do occur, then the fish may display a number of symptoms. Perhaps the most obvious of these is behaviour known as "gasping", where they hang near the surface, where oxygen levels are highest. In ponds, this behaviour is often seen first thing in the morning, when oxygen levels have been depleted overnight. In addition to this, the fish may appear generally lethargic and stop eating. Usually, the largest fish will be the first to suffer, along with any particularly oxygen-sensitive fish, such as Orfe and Sturgeon.

When low oxygen levels are suspected, you should follow this procedure:

Add TetraPond WaterSafe to the pond and the use a hosepipe to disturb the surface of the water. This will help to restore oxygen levels in the short-term. Performing a 30% water change, using TetraPond Watersafe, or AquaSafe, will also help to cool the water down and increase its oxygen content, in either a pond or tank. Install an additional pump in the pond or tank, to disturb the surface of the water and increase oxygen levels. This will help in the long-term.

Make sure that your pond or tank is sensibly stocked and that any algae problems are resolved. Check the sediment levels, and remove it if it is building up.

By taking the necessary precautions to ensure that oxygen levels are maintained, you will find that your fish remain much healthier and appear far happier.

Last updated January 2005