Fish, Tanks and Ponds

Fish, Tanks and Ponds
A comprehensive guide to fish

Fish-Keeping and Oxygen

Giant danio Devario aequipinnatus
Giant danio Devario aequipinnatus


Oxygen is vital to life in the aquarium, without it or if there isn't enough of it quite simply the fish will all die quite quickly. There are many factors which have an effect on the level of dissolved oxygen some of which have a big impact on aquarium or pond water and other factors which are often quoted but have little impact. But what is oxygen and what does it do.

Oxygen in water

As most people know water is a molecule made from two hydrogen atoms and one oxygen atom (H2O). The oxygen which forms part of the molecule is not available to fish to breathe, only dissolved oxygen (DO) is available to the aquarium or pond inhabitants. The amount of dissolved oxygen is affected by several things:

  • Air pressure.
  • Salinity.
  • Temperature

Of these salinity and temperature have the biggest impact. Air pressure plays almost no part in this unless you are at high altitude.

Air pressure: At sea level the range of air pressure is so small that it has little or no impact on the dissolved oxygen level in water.

Salinity: Water is a very good solvent it will dissolve all kinds of things but it has a finite ability to do this. In simple terms the more dissolved solids that a water carries the less room there is for oxygen. This why sea water hold much less oxygen than fresh water.

In the fresh water aquarium and pond where fish stocking levels are far higher than those found in nature and where the level of dissolved organic compounds, phosphates and nitrates build up relatively quickly is naturally going to lead to lower dissolved oxygen levels just as in sea water.

Temperature: Water will hold less free oxygen as the temperature rises. Normally this doesn't affect the livestock in an aquarium as long as they are kept within certain temperature ranges which they have evolved to live in. If the temperature rises to high then two things will happen.

1, The level of dissolved oxygen will fall.
2, The fishes metabolism and demand for oxygen will rise.

It is obvious that this situation could very easily cause problems for the fish if left unrectified. Some fish which are sold as tropical fish actually come from temperate regions rather than true tropical regions and could be classed as being semi tropical. Some corydoras, barbs and danios may all fall in to this category. If these fish are kept at the high end of the tropical temperature scale they will not thrive and it is likely that their lives will be shortened as a result.

Factors causing low oxygen levels

Stratification: where the water forms in to layers can result in the lower region becoming low in dissolved oxygen because it doesn't mix with the layer high in oxygen near the surface and it has no surface where oxygen is able to dissolve. This shouldn't occur in an aquarium or small pond.

Bacterial blooms: Can occur in new or in neglected aquariums and ponds. They are caused by a sudden glut of nutrients which then allow the bacteria to multiply rapidly. Bacteria like most living things need oxygen and when in great numbers like this they rapidly consume all the free oxygen in the environment unless steps are taken to increase the oxygen level. Bacterial blooms normally only last for a day or two before the nutrients are used up and the bacteria return to their normal background level.

Algae bloom: Problematic algae blooms are more likely to occur in ponds rather than aquaria. Thick blanket weed can grow at an alarming rate and green water can develop in hours under favourable conditions. The problem occurs when conditions become less favourable and the algae begins to die back. The nitrifying bacteria which oxidise ammonia in to nitrate consume vast amounts of oxygen to do so. When so much algae dies it can deplete the water of all its free oxygen very quickly. This is one reason why it is important to physically remove as much blanket weed as is possible before treating it with an algaecide.

Stocking levels: Fish stocking levels are a major factor in the level of free oxygen in the water. Fish are large active creatures with a high oxygen demand. More fish equals more waste, more nitrates too which means much higher levels of maintenance to keep things running smoothly.

If a pond or aquarium is over stocked then the oxygen can be consumed faster than it can be replaced. If a pond or aquarium is stocked to the limit where it then has to rely on the equipment to maintain sufficient levels of dissolved oxygen and then that same equipment fails the whole system fails.

If you stock to a level where you are relying on the equipment then it is prudent to keep spares of all the essential equipment to avoid a disaster. It is far better to stock sensibly where in the event of a hardware failure the system would carry on working at least in the short term (such stocking levels are generally more pleasing to the eye too).

Important oxygen levels

Most fish require the dissolved oxygen level to be in excess of 6mg/l in order to be able to function normally. If the level of dissolved oxygen falls to far below this the fish will be seen gasping at the surface in order to get at the oxygen as it is dissolved.

Below is the level; of dissolved oxygen in clean fresh water at ground level.

  • Temp 0°C (32°F) - 14.6mg/l of dissolved oxygen.
  • 5°C (41°F) - 12.8 mg/l of dissolved oxygen.
  • 10°C (50°F) - 11.3 mg/l of dissolved oxygen.
  • 15°C (59°F) - 10.1 mg/l of dissolved oxygen.
  • 20°C (68°F) - 9.1 mg/l of dissolved oxygen.
  • 25°C (77°F) - 8.3 mg/l of dissolved oxygen.
  • 30°C (86°F) - 7.6 mg/l of dissolved oxygen.
  • 35°C (95°F) - 7 mg/l of dissolved oxygen.
  • 40°C (104°F) - 6.5 °F

The salinity in clean sea water has a dramatic effect on the levels of dissolved oxygen as shown below.

  • 0°C (32°F) 11.2mg/l of dissolved oxygen.
  • 5°C (41°F) 9.9mg/l of dissolved oxygen.
  • 10°C (50°F) 8.8mg/l of dissolved oxygen.
  • 15°C (59°F) 7.9mg/l of dissolved oxygen.
  • 20°C (68°F) 7.2mg/l of dissolved oxygen.
  • 25°C (77°F) 6.6mg/l of dissolved oxygen.
  • 30°C (86°F) 6.1mg/l of dissolved oxygen.
  • 35°C (95°F) 5.6mg/l of dissolved oxygen.
  • 40°C (104°F) 5.3mg/l of dissolved oxygen.

Any dissolved solids will reduce the dissolved oxygen level by this amount, not just salt. Dirty polluted water will hold even less dissolved oxygen than sea water and this is one reason why water changes are so important because they keep the level of pollutants in check.

How oxygen enters water

Surface area and turbulence: Oxygen will dissolve in water where the two meet which is normally at the surface. CO2 is even more soluble in water than oxygen but it is less stable and agitation will liberate it back in to the atmosphere leaving more room for the oxygen. Movement at the surface also helps the exchange of gasses meaning less CO2 and more oxygen. In short, having a moving or agitated water surface will aid the oxygen to dissolve in the water.

Air stones: Air stones liberate lots of tiny bubbles. Bubbles have a relatively large surface area and so lots of tiny bubble greatly increase the area where air and water meet. This allows more oxygen to dissolve in to the water more quickly than it would other wise have done.

Photosynthesis: Plants actively take in CO2, use the carbon to form sugars and liberate the oxygen as a by product. But there needs to be a considerable plant mass which is healthy and actively growing for this to have much effect.

Venturi device: Venturi's are particularly useful because they are normally placed just after the filter. This means the water which has just been through the filter where the biological filtration will have depleted the free oxygen is mixed with air in a very turbulent fashion ensuring that the water becomes high in dissolved oxygen when normally without a venturi this water would be returned to the pond/aquarium with very low levels of dissolved oxygen.

Fish and oxygen

gill fillament
A close up view of a single gill filament

Fish are very efficient at extracting dissolved oxygen from water through their gills. There are two ways in which water passes over the gills.

Ram ventilation: Where water enters through the fish's open mouth as it propels forward and then exits through the gill slits. This method is used extensively by sharks and by some fast moving species like tuna.

Active ventilation: Where water is actively pumped in through the mouth and out through the gills even when the fish is not swimming forward. This is the method used by the overwhelming majority of pond and aquarium fishes.

As water flows over the gill filaments gas exchange occurs and oxygen is absorbed in to the blood while carbon dioxide is expelled in to the surrounding water.


Oxygen is vital to life in the aquarium, without it or if there isn't enough of it quite simply the fish will all die quite quickly.

There are many factors which have an effect on the level of dissolved oxygen some of which have a big impact on aquarium or pond water and other factors which are often quoted but have little impact.

The minimum level of dissolved oxygen varies from fish to fish but generally it is as follows.

  • Cold water fish 6 mg/l
  • Tropical fish 5mg/l
  • Marine fish 5 mg/l

If the levels fall below this for prolonged periods of time it is likely that the fish will fall victim to various stress related illnesses such as fin rot, white spot or other opportunistic bacterial infections due to their immune system not working properly.