Dissolved oxygen and pond fish

Dissolved Oxygen Concentration (DOC) is the term given to describe the amount of oxygen that is dissolved in pond water.

Dissolved oxygen is measured in terms of how much oxygen (by weight – mg) is dissolved in every litre of water. There are many factors that will affect DOC in a koi pond, some of which we can control, others we cannot, but can respond in a number of ways to guard against problems.

Of course, dissolved oxygen is important to all life in a koi pond, not simply the fish, and they all have a demand on the finite supply of oxygen dissolved in the pond water. The number of oxygen consumers in a pond will determine whether the dissolved oxygen concentrations in the pond are likely to be a problem or not, with a larger density putting a greater strain on the resource.

Oxygen consumers in a koi pond.


In relation to the activities of bacteria and other microscopic pond organisms, the oxygen demand by koi is not the greatest (but this will vary with stocking density). Compared to many other fish species, koi are relatively tolerant of low dissolved oxygen concentrations (which is understandable, being a fish from slower moving warm waters). The range of DOC in a pond will run from a minimum (0mg/l) to a maximum (about 15mg/l), where even the supply of additional aeration would not change the DOC. A trout requires a high DOC of about 12mg/l while the more tolerant koi can only require a much lower 6 mg per litre. Even so, below this, for short periods of time they can gasp at the surface to gain their required oxygen.

Koi are equipped with gills and having a very fine and delicate structure are used to absorb the oxygen that passes through the gills, dissolved in the water. They are very efficient at removing oxygen, absorbing 80% of the oxygen that speeds by their gills in the water (compare that to about 20% efficiency of our lungs). Furthermore, koi are able to pack the oxygen from 100 ml of water into 4 ml of blood, quite a feat of bio-engineering.

Bacteria / Fungi …

The majority of bacteria in a pond are aerobic (requiring oxygen) being found in abundance in the filter. The activity (and oxygen demand) of a bacterial population will vary, depending on the water temperature and the amount of food that is present for them to break down in the water.

The bacteria involved in breaking down ammonia into nitrites and then nitrates require oxygen. The greater the supply of ammonia and therefore nitrites, the greater the bacterial activity and oxygen demand. A further group of bacteria, known as heterotrophic bacteria, feed on organic matter, (both soluble and solid), that is present in the pond. As solids gather in a settlement chamber or excess food is allowed to float uneaten, the oxygen demand of these heterotrophic bacteria will rise as they gorge themselves on a banquet of food and fish waste.

Invertebrates …

The more mature a filter becomes, the greater the diversity of the invertebrate life that will colonise. A range of hungry critters from the virtually microscopic to the easily visible will all demand oxygen as they break down organic matter. Again, the more food there is to break down, the greater the demand on the pond’s oxygen reserves.

Plants …

Plants, both microscopic algae and larger plants are oxygen consumers, day and night. Plants, like fish and all the other pond life respire 24 hours a day, using up oxygen. Fortunately, the plants, during the day, also add oxygen to the pond’s oxygen budget in excess of what they require in the day.

Oxygen Budget …

However, as most koi ponds are sparsely planted, particularly with oxygenating plants, then there is likely to be little oxygen added courtesy of the plant life. Furthermore, as fish stocking, and feeding rates are in excess of what mother nature would decree, there is likely to be insufficient oxygen entering the pond passively, by diffusion from the atmosphere. This can soon lead to an oxygen debt, where the demand and consumption of oxygen is in excess of the rate at which it is being added.

Factors that can cause the DOC to drop:

Biological Factors …

The demands on the oxygen dissolved in the pond can be extreme. If feeding rates are high and fish are at their most active, then koi will be consuming oxygen at a high rate, as will the populations of bacteria and invertebrates breaking down the ammonia and undigested organic matter excreted by koi.

Plants in the daytime are natural oxygen producers, exceeding their own oxygen demand. At night however, they still use oxygen but are no longer producers. This can lead to DOC dropping at night, especially if your pond is suffering from an algal problem. If algaecides are used, which can cause algae to die off, this too can present a problem as it attracts considerable bacterial activity all of which consume D.O.

Climatic Factors …

Temperature has a twofold effect on DOC. Firstly, as temperature increases the ability of water to hold dissolved oxygen decreases. Compounding this phenomenon is the effect that temperature has on the metabolism and oxygen demands of all aquatic life in a pond. As water temperatures increase, koi will consume more food, excrete more ammonia which attracts more bacterial action in the filter. So just when a ponds can ill afford too many demands on its oxygen, the oxygen requirements within a pond, including koi reach their highest levels of the year. It is essential to step in and add more oxygen.

Chemical Factors …

Pond treatments, both directly and indirectly will cause oxygen to become limited in a pond. Any solute (dissolved substance added to a pond will limit a pond’s ability to hold as much oxygen as was possible prior to the addition of a solute. So when adding any treatments, additional aeration is always recommended.

Indirectly, many treatments can lead to an increase in organic matter that will be broken down. For example, algicides may lead to dead algae settling out in the filter, or other treatments can kill off bacteria, or lead to an increase in mucus production. All of these will eventually be broken down by oxygen-hungry bacteria.

Effects of low DOC on fish …

Low DOC has very obvious affects on koi. If there is insufficient oxygen dissolved in the pond water, koi will respond in the short term, by becoming lethargic, swimming slower than normal, and perhaps hanging towards the surface.

If the condition persists or even deteriorates, koi will gather at water inlets or below a water fall and even start gasping air from the surface. By this stage they will already have become stressed, making them very susceptible to disease. Whenever I have taken a mucus scrape from a gasping fish, I have found an abnormally high density of protozoan parasites, busy capitalising from the koi’s momentary lapse of defence.

A low DOC is also likely to affect koi health indirectly, by adversely affecting the biological filter. The functioning of bacteria that are responsible for the breakdown of toxic waste is directly proportional to the dissolved oxygen concentration. Low DOC, will lead to poor filter performance, and a peak in ammonia/nitrite may well lag behind such an incident.

Detecting and measuring more precisely …

A DOC deficit is only likely to occur at warmer temperatures, and a change in koi behaviour will be the first signs of a problem. A more precise method of quantifying the problem (or the margin of safety on a hot day) is with a digital DO meter. In an instant, they will show the DOC and more advanced units will indicate the degree of saturation by simultaneously measuring the water temperature. Even more advanced is a permanent DO monitoring system which will sound an alarm should DOC levels drop dangerously low. Such units are common place in intensive aquaculture and can also be used to detect either a pump or air pump failure due to the associated drop in DO.

Immediate action …

An acute low DO problem can occur through equipment failure, through unforeseen circumstances, or overlooking the likely effects of a pond treatment. In either case, once low DOC is detected, remedial action should be swift.

Agitate the water’s surface short term by pouring buckets of pond water back into the pond. This can alleviate a short term problem while you think of a more permanent solution.

Spray a hose into and across the pond, simulating rain. This will cause DOC to rise as an emergency measure, and the cooler tapwater will allow pond water to hold more oxygen.

Preventing a low DOC is far better than responding to a problem, and there are many aeration strategies that will solve the problem:

Air Pump and Diffuser …

Air is the most effective when it is pumped to the bottom of a pond or filter chamber and dispersed into as fine bubbles as possible. The deeper a pond is, the greater the head of pressure an air pump has to displace before managing to deliver air to a diffuser. Hi-blow pumps are the koi keeper’s standard, delivering good volumes of air under pressures capable of reaching depths of 8 feet. Recognising that both fish and filters are oxygen consumers, aeration should be added to both the pond and biofilter chamber. Avoid aerating settlement chambers as the mixing action created by the airstones will interfere with the settlement action.

The golf ball type of air diffuser is very easy to install and cost effective. Smaller bubbles can be achieved using different diffusers, such as leaky pipe, micro-bubblers and air domes. The latter converting a bottom drain into a diffuser where the airline can be trailed, out of sight, up the water pipe and into the filter.

Injection …

Venturis inject air into a stream of water as it returns under pressure from the pump. Useful at creating a mixing effect in the surface layers of the pond water, but not regarded as effective as an air pump and diffuser system.

Waterfalls ….

These mix air and water as it cascades down the waterfall and enters the surface. In similar fashion to the venturi, a waterfall’s beneficial effects are limited to the upper layers of pond water.

Do’s and Don’ts to avoid a low DOC:


Try to aerate a pond as a matter of course, endeavouring to aerate down to the pond bottom.

Add additional aeration to the biofilter chambers, it will enhance bacterial colonisation.

Clean out settlement chambers regularly. Even when organic matter is removed from the pond, it still attracts oxygen demanding bacteria in the filter.

Add additional aeration during any pond treatment or medication.


Assume that if your fish have survived the hot day without additional aeration, they will survive the night. Night time is when DOC can really begin to drop, so continue to aerate.

Continue to aerate below 12oC. Allow fish to settle in warmer deeper water. Aerating will simply mix up all layers of the water, losing the benefit of having such a deep pond.

Over stock. This will lead to a doubling of demand on oxygen from the koi and the bacteria breaking down their waste.

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Kill blanketweed and string algae.