Do fish need oxygen?

Novice fish keepers often ask the question, do fish need oxygen? It is not long before they learn that the answer is an emphatic yes.

Oxygen is often described as the fuel of life and, as such, it is critical to all aquaponic systems. It is an essential ingredient of respiration , a process that releases life giving energy. Consequently it is required, not only by the fish, but also by the plants and filter bacteria.

Lack of oxygen is the number one killer of fish and characterised by fish seen gulping at the surface. Wildlife ponds often suffer from oxygen depletion during hot summer nights.  If plant roots are starved of oxygen, by water logging for instance, they too will die. In the absence of oxygen filter bacteria are unable to breakdown ammonia to less harmful nitrite and nitrate. All these processes are utterly dependent upon adequate levels of oxygen.

Oxygen is a critical requirement for respiration and, as such, all fish will quickly die if adequate levels are not maintained. This can be as a result of sewage pollution, algal collapse, thunderstorms or pump failure.

Why plants need oxygen

Respiration occurs continuously in all living cells. It is only in the presence of light that plants are able to produce oxygen over and above that needed for respiration. Root cells do not have access to light and, as a result, have a continuous requirement for oxygen. Some plants, such as rice, are able to supply this directly through a series of air channels. On the other hand most plants require oxygen to be made available at the root zone.

Consider backup seriously

All intensive growing on systems should really have a back up system for maintaining safe dissolved oxygen levels in the event of a power cut. Removal of solids and ammonia can be delayed for short periods but lack of oxygen is a real killer. Some of the fish farms I have worked on were so heavily stocked that the fish had around 20 minutes before they were all dead following a system failure. Not inducive of a good nights sleep!

Species Variation

Different species of fish have different tolerances to low oxygen. A factor that enables them to exploit different habitats. Trout require high oxygen levels and thrive in cool, running streams and rivers. Carp and catfish, on the other hand, can exist in still, warm ponds with much depleted oxygen levels. This is just as well for carp because they can consume a lot more oxygen per kg. body weight than trout at these higher temperatures. This is not generally realised but is vital when designing aeration systems for growing on warm water species. Most research indicates that trout consume around 200 – 250 mg O2/kg/hour. I regularly record carp consuming three times as much at 700- 800 mg/kg/hour under optimal growing conditions.

Oxygen Solubility

Unfortunately, water is an oxygen poor environment, unlike ammonia and carbon dioxide. It is very difficult to dissolve in ponds. Water only contains 5% of the oxygen that the same volume of air contains. The solubility of oxygen in water is very much dependent upon the fish tank temperature. Warm water contains a lot less dissolved oxygen than cold water. The table below shows the mg. oxygen that will be present in water that is fully saturated with air at various water temperatures.

Temperature C. Oxygen Concentration at 100% air saturation
0 14.6 mg/l
5 12.8 mg/l
10 11.3 mg/l
15 10.1 mg/l
20 9.1 mg/l
25 8.2 mg/l
30 7.5 mg/l

Remember also that fish consume far more oxygen at higher temperature. There is a reduced availability coupled with a higher demand meaning that oxygen provision can quickly become critical.

Factors affecting oxygen consumption

In addition to water temperature there are a number of other factors that will affect just how much oxygen your fish will consume. It’s well worth bearing these in mind as you may need to manage them quite closely, especially close to harvest time. They will also have a bearing on the design of your aeration system. The three factors to consider are as follows;

  • Fish size – smaller fish consume relatively more oxygen than larger fish
  • Feed rate – the more feed given the more oxygen required
  • Stress– the higher the stress level the higher the oxygen consumption.

Monitoring dissolved oxygen

If you are serious about maximising your fish, and plant, production regular monitoring of dissolved oxygen levels is essential. If you are less keen on an intensive fish system then monitoring is not so vital. Many people will fall into the latter group.

Readings can be expressed in one of two ways. Either as a percentage of air saturation, or as mg per litre (mg/l) The later is exactly the same as parts per million (ppm).

Dissolved Oxygen Meters

Without doubt, electronic dissolved oxygen meters are the most convenient method of analysis. They are easy to set up, do not require reagents and give instant results. They are also able to express the results in either % air saturation of mg/l. Unfortunately they are quite expensive. A fact that may explain why so few hobbyists possess one. There are much cheaper drop test kits available but I have found them to be far less reliable and much more of a hassle to operate. Cost of reagents also needs to be considered. Personally I record percentage air saturation as I am usually operating at a stable temperature and calibration is slightly more straight forward.

This is the dissolved oxygen meter that I have selected to use. I have used many different designs over the years at vastly different prices. So far, I am very impressed with this model and represents excellent value for money. I will post a more complete review once I have thoroughly tested it.

It is possible to set these meters up to give a continuous reading. I tend to dislike this method as regular calibration and cleaning of the probe is a pain. I prefer to calibrate the meter once, then check manually at several points around the system. It is extremely useful to know just how much oxygen is being consumed by the fish, the plants and the filter bacteria. Knowing this information enables us to optimise water flow rates.These should be the minimum required to promote optimum respiration.

As far as a means of triggering a backup aeration system goes I much prefer the reliability offered by measuring water levels in a header tank. By and large if the pump is operating the water level in the header is high indicating that there is sufficient water flow into the fish tanks.

Aeration Systems

The Role of Photosynthesis

Oxygen dissolves in water through two main pathways. The first is through the action of light on underwater plants, including algae, through the process of photosynthesis. This produces pure oxygen but only occurs during daylight hours. At night the plants respiration processes dominant with the result that carbon dioxide is the main gas produced. This action of photosynthesis can often lead to very high variations in dissolved oxygen levels over a 24 hour period. Natural ponds can swing from 130% air saturation to dangerously low 40% saturation every day in late summer.

The Role of Diffusion in Aeration

Plants do not play any significant role in aerating intensively stocked ponds and tanks. Instead we have to rely on diffusion of oxygen from the air into the pond water. Levels therefore do not exceed 100% and are usually considerably lower. The process of diffusion forms the basis of most aeration systems. Lack of oxygen is the first factor that limits the number of fish that a pond will hold. Fish importers will inflate polythene bags with pure oxygen to ensure they live.

Diffusion is dependent upon the surface area of the air / water interface. Ponds with large surface areas tend to be able to support more fish than deeper ponds of the same volume. Bubbling air into the water is simply a convenient way of increasing the air to water ratio. It is by far the most common method of aeration and is usually entirely satisfactory for most aquaponic systems. If however, you intend to grow fish commercially at high stocking rates more efficient aeration techniques are available.

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