Garden pond dissolved oxygen

Oxygen, why is it important in the garden pond?

Most fish keeper’s are well aware that the pond or aquarium water should be routinely tested for the presence of pollutants such as ammonia and nitrite but very often the oxygen content is overlooked. Oxygen is an essential gas, which is readily soluble in cold water but becomes increasingly insoluble as the water temperature rises and this can have important consequences for the fish, particularly in the garden pond during the summer months. Breathing air is really quite easy and requires very little energy but water is much more dense than air, so fish have to apply some effort in extracting oxygen from water. Using energy, implies the fish requires oxygen just to breathe, so fish must consume this vital gas to extract it from the water. When dissolved oxygen levels are low it can mean the amount of oxygen the fish are using to breathe is greater than that found in the water, which leads to what is termed ‘a negative oxygen debt’. If the dissolved oxygen content of the water is not rapidly increased, the fish will become stressed and subject to secondary disease and infection, or even die.

What affects the oxygen level in the pond or aquarium?

The numbers, size and species of fish which are stocked has an immediate impact on the dissolved oxygen content of the water. The more fish in the system, the greater the competition for the available dissolved oxygen and big fish, use much more oxygen than small fish. Often one of the classic signs of low dissolved oxygen, is when the largest fish apparently die for no reason. Some species of fish are much more active than others and as a consequence have a greater oxygen requirement and these are affected first.

Submergent aquatic plants also have a significant effect on the dissolved oxygen content. During the hours of daylight, all plants produce nutrients using the energy from sunlight and carbon dioxide, releasing oxygen as a waste by-product in a process known as ‘Photosynthesis’. It is often not appreciated that all plants, including aquatic species also respire, which means to say that they use oxygen and give off carbon dioxide, exactly the same as ourselves. At night,

photosynthesis stops and the plants actively respire, making use of the nutrients they have produced during the day time. The fact that plants respire at night means they compete with the fish but are much more efficient at extracting oxygen from the water. This can lead to dissolved oxygen levels crashing overnight, especially in the garden pond, during the summer months when oxygen is relatively insoluble in water and causing the largest fish to be found dead in the morning.

Submergent aquatics planted in the garden pond are popularly known as ‘oxygenating’ species. These varieties have earned their name because the oxygen is given off as a discrete bubble, which forms on the tips of the leaves and is readily visible. There is no doubt that oxygenating plants play an important role in aerating the water during the daytime but providing the growth is not profuse. There are no hard and fast rules about how much established aquatic plant life should be growing, as it depends on the size and volume of the individual pond. It can help to regularly trim back submergent and oxygenating species, particularly through the summer, when the growth is vigorous.

It is also very important to realise that aquatic plants play a very important role in removing the ammonia waste produced by the fish and that the captive aquatic environment is a balance. In the wild, the amount of aquatic plant is continually cropped by the fish stocks, ducks and geese, which helps to maintain this balance.

Bacteria and other micro-organisms have a very important function in the filtration system, in breaking down the harmful fish waste ammonia, to nitrite and then nitrate. The chemical conversion of ammonia to nitrate uses oxygen but the bacteria and other ‘bugs’ in the filter system also need to breathe oxygen. This means the filtration system also needs oxygen, in fact it is the biggest consumer in the aquatic system! Bacteria find it extremely easy to extract the oxygen from the water and this means they take as much as they need and the fish get what is left over, which may not be very much! It is therefore important to ensure the filtration system is adequately aerated as this can increase the oxygen availability for the fish.

How do I know if there is enough oxygen for my fish?

It is commonly thought that if there is not enough oxygen in the water, then the fish will be seen gasping at the surface but this is a last resort means to breathe. The first indication there may be a dissolved oxygen problem in the water is when the fish become unusually lethargic and stop feeding. As oxygen levels decrease, the fish do not have enough energy to swim and feeding utilises yet more oxygen. Often it is recognised the fish have a problem at this stage but frequently some form of medication is added to the water and this can actually cause the oxygen level to drop even lower, leading to a number of mortalities. This can lead to the mistaken conclusion the fish were suffering from some form of disease. In terms of managing any aquatic system, it is always advisable to increase the aeration when any fish start to behave abnormally and before adding any form of medication to the water.

Increasing the aeration will certainly make the environment more comfortable for the fish, even if the dissolved oxygen

level was satisfactory. With regard to improving the aeration before adding medication, this will allow for any depletion of the oxygen level caused simply through a chemical reaction.

There are test kits available for monitoring the oxygen levels in aquatic systems, the most well known is probably the TetraTest Dissolved Oxygen Kit which relies on matching the colour of the reagents with that on the package to gauge the oxygen content. One kit manufactured by New Technology adds the final solution in a drop wise fashion and the number of drops equates to the dissolved oxygen concentration in the water. Both kits are very easy to use and readily available at

How much oxygen do my fish need?

The recommended minimum dissolved oxygen requirements are as follows:

Cold water fish – 6 mg per litre

Tropical freshwater – 5 mg per litre

Tropical marine – 5 mg per litre

It is worth bearing in mind these values are minimum requirements for healthy growth, tissue repair and reproduction.

Most fish species will tolerate a drop below these minimum values for a short period of time, probably the cold water species are likely to tolerate a lower level than tropicals. However short the period of time the oxygen level drops below the required minimum it will cause the fish to become stressed. It may take the fish several days to recover from a short term oxygen depletion but where the levels are persistently low, an assortment of stress related diseases such as fin rot and white spot may occur.

I have heard that adding too much air to the water can cause gas bubble disease?

Gas bubble disease tends to be confined to cold water systems, particularly garden ponds but usually in the winter months when the water is cold but it is not a common disorder. Air is readily soluble in the cold water but gas bubble disease occurs when air is drawn into solution under pressure, such as through a tiny fracture or pin hole in pipe work, acting rather like a small venturi device. Gas bubble disease can also be caused where cold water is heated rapidly, trapping the air and causing supersaturation of the water. Nitrogen comprises the greater part of the volume of air and it is usually this gas which leads to the condition called gas bubble disease in fish.

Gas bubble disease shows remarkable similarities to decompression sickness or the bends which affects human divers ascending through the water too quickly. Affected fish can show spasmodic cramping of muscle tissue but the classic symptom is the presence of tiny bubbles of gas in the finer tissues of the fins and around the eyes. Providing the problem is rectified, the fish will recover over a period of time, some tissues getting rid of the air more quickly than others.

Where a pond is affected with gas supersaturation, the bubbles can be seen breaking the water surface of an affected pond, rather like the top of a carbonated drink, when placing a hand into the water, tiny bubbles immediately form, which are readily visible. As strange

as it may sound, the cure for a pond affected with gas bubble disease is to splash the water vigorously, such as allowing pumped water to hit an obstruction such as a piece of wood, allowing the air to gas off.

Kill blanketweed and string algae.