Dissolved Oxygen and pH

Syd Mitchell's picture
Submitted by Syd Mitchell on Sun, 05/06/2018 - 09:12

By: Syd Mitchell

This article explores oxygen and what it has to do with pH, and why pH is important to know.

There are six pond parameters that should be regularly tested, they are ammonia, nitrite, nitrate, pH, KH, and oxygen - which one do I rarely test with a meter or a test kit?

The answer is oxygen but although the reasoning is fairly straightforward, it has to be clearly understood that this isn’t an excuse to become complacent about dissolved oxygen.

Are oxygen tests necessary?

Oxygen tests can be fiddly and/or time consuming.  Every time I look at my pond, I see air pouring into it from a large flat ceramic air diffuser.  There is also a small waterfall.  When I look into the filter bays, I see the amount of air being fed to the aerated K1 bay is so great that the K1 isn’t just moving around it looks as if it is really “boiling”.  The pond isn’t overstocked and it is always kept clean and free from anything that might rot down or decompose and use oxygen in the process. Under these conditions, whenever I have tested the oxygen level, it is always almost at “saturation point”.

Saturated water

The oxygen saturation point is the maximum value of oxygen that water could hold and this value couldn’t be made to go any higher even if more air stones were to be added.  It varies considerably with temperature and also it reduces slightly according to the height above sea level although this is often ignored since it won’t make much noticeable difference except for Koi ponds at very high altitudes.

Temperature °C








Oxygen mg/L








Above - Saturation values of oxygen in fresh water at sea level at various temperatures.

In reality, although many articles refer to the importance of reaching 100% oxygen saturation, in practice 100% is difficult to achieve in a pond where Koi are continuously breathing some of the oxygen as we are adding it. Also many normal pond treatments will slightly reduce the capacity of the water to hold oxygen.  However, 90% should easily be achievable.

For this reason, it is sensible to initially check oxygen levels for a while to make sure that well over 90% of the oxygen saturation value is being achieved, but although my life revolves around testing water parameters, I don’t waste time taking tests when there is no reason to do so.  When I can see at a glance that the aeration that produced high oxygen values in the water hasn’t changed, I consider it unnecessary to test for dissolved oxygen with a test kit because I can see that the fish are getting plenty of oxygen, every time I walk past.  This is the part that must be understood; this only applies to ponds where there is continuous high level of aeration.

The answer to my question should not be interpreted as: - “Don’t bother to check dissolved oxygen”, but as: - “If your own pond is equally highly aerated, you should test occasionally to make sure that at least 90% saturation value is being achieved but, as long as that rate of aeration remains the same, there is no need to repeatedly test what is obvious every time you look at your pond”.

Too much aeration?

It is sometimes said that a pond can be too well aerated or that air-stones can be placed too deep and that this can cause fish to get “the bends”.

The bends is nothing to do with the shape of fish. The bends, more properly called decompression sickness, is a common hazard for scuba divers who use compressed air to breathe underwater. At depths, the compressed air they are breathing dissolves into their blood in the same way that the air we normally breathe on land dissolves into our blood. When these divers surface from depths, they have to do it slowly or in stages to give the dissolved gasses in their blood time to disperse. If they surface too quickly before this has had time to happen, the gasses expand and form little bubbles which can lodge in their joints or other blood vessels causing pain, paralysis or even death. This cannot happen to fish in a pond.

The carp from which Koi have been bred have evolved in lakes that are considerably deeper than a Koi pond. They swim from the depths to the surface and back again and they don’t suffer from the bends. If carp were affected by the bends, the species would not be here today, they would all have died out over evolutionary time scales. The idea that Koi can get the bends is probably a misunderstanding of a similar effect, sometimes called "gas bubble disease" which can result from pond water becoming supersaturated with air.

Supersaturated water

Supersaturation can occur where air is introduced into water at a high pressure. Normal pond air pumps don’t generate a high enough pressure to cause this effect so you can put air stones at any depth where they will still work, but it is possible that supersaturation could be caused by faulty plumbing.

If there is a pin-hole leak such as a badly glued joint on the pipe-work leading to the suction (input) of a reasonably powerful pump, when the pump is running, water won’t leak out but air will be drawn in though the pin hole. As each bubble of air goes through the pump, it will be compressed due to the increased water pressure and it will dissolve. If it then goes straight back to the pond, the dissolved bubbles of air will remain compressed in the pond water for several seconds at least, maybe for a few minutes, depending on temperature and aeration.

Above - Bulging eyes (exopthalmia) is a typical external symptom of gas bubble disease

If a fish takes compressed dissolved oxygen or especially nitrogen into its body via the gills, it can then expand and form bubbles in body tissues or the bloodstream. In severe cases this can kill the fish, although I have personally only seen one case in which a fish died, and that was in a very large tropical marine tank where a replacement powerful pump had been badly plumbed in with a pin-hole leak on the pipe-work as described above.  An easy way to tell if a pond is supersaturated is to put your arm into it. If little bubbles immediately start to form on the tips of the hairs on your arm, you can suspect supersaturation and should check the plumbing leading to your pond pump.

Plants in very shallow water in strong sunlight can also cause the water immediately around them to become slightly supersaturated with oxygen, but with the exception of a pond that is severely over-grown with a dense mat of oxygenating plants or blanket weed, I doubt that normal plants could supersaturate pond water with oxygen to a sufficient degree to harm Koi in a pond.

Air stones don’t have to be right at the bottom for good aeration, due to the limited pressure that the air pump can provide they may work better if they are raised slightly, but they will not be able to supersaturate the water and cause gas bubble disease regardless of the depth they are placed at.

 Too little aeration?

If oxygen tests with a meter or a test kit show that the dissolved oxygen level isn’t high enough or, worse still, if fish are showing the symptoms of insufficient oxygen such as gasping at the surface etc there is one simple remedy - increase aeration immediately. More air-stones and/or a more powerful air pump are the usual ways to do this.

Beaten by a fish

The answer to the second question; “why is pH the most important test we should make on a pond?” requires understanding some basics about fish physiology, but let’s start with what we already know.

Most, if not all of us, know that the means by which oxygen from the air we breathe into our lungs is carried around our bodies is by something called haemoglobin. Haemoglobin is a molecule in red blood cells that has some very clever properties that allow it to pick up oxygen from our lungs. Our hearts pump the oxygenated blood cells around our bodies and, in that way, oxygen can be carried to the tissues that need oxygen. In scientific circles this is called the oxygen transport mechanism.

Since humans evolved from mammals that ultimately evolved from fish, it should come as no surprise that we share a similar oxygen transport mechanism except that fish had it long before we evolved from them.  Our oxygen transport mechanism is a very efficient way to get oxygen all though our bodies but fish had it first; they beat us to it by hundreds of millions of years.

The importance of pH

The difference between the fish oxygen transport mechanism and ours is that, whereas we use lungs to get oxygen into our blood, they use gills and this is why pH is so important to them. To understand why, we need to understand a little about how haemoglobin works.

As haemoglobin passes though the gills, it picks up atoms of oxygen from the water but it doesn’t hold on to them too tightly, its grip is weak enough that, as it is pumped through the body, and passes a cell that is in need of oxygen, the cell can pull the oxygen away and use it.  But haemoglobin has more tricks up its sleeve.  Cells that use oxygen also produce a waste product, carbon dioxide, that they need to get rid of in order to make room inside for the new oxygen that they need to continue functioning. This is haemoglobin’s second trick, as it allows the cells to take the oxygen it is carrying it also picks up the waste carbon dioxide and carries it back to the gills.  Its third trick is to dump the carbon dioxide out through the gills and take on new oxygen ready to be pumped back round the body in order to endlessly repeat this process.

How does it know how to do this? It doesn’t, the whole process is automatic, and it is controlled by an enzyme called carbonic anhydrase and the fact that the blood has a pH of about 7.4 but which varies up and down slightly according to whether it is carrying oxygen or carbon dioxide.

This diagram above shows a simplified representation of the Koi respiration cycle.  The heart pumps oxygen-depleted blood through the gill where haemoglobin releases carbon dioxide into the water and absorbs oxygen.  Oxygenated blood then flows throughout the body where haemoglobin gives up oxygen to the individual cells in exchange for carbon dioxide. The oxygen-depleted blood is then pumped by the heart back through the gill where the respiration cycle begins again.

To make the oxygen transport process work efficiently, the fish’s blood chemistry must change slightly according to the pH of the water it is in.  Different fish can control their blood pH according to their usual environment range. Marine fish have evolved in an environment where the pH is fairly constant and so have limited ability to cope with pH changes. Carp evolved in a less stable environment and are happy in any pH in the range 7.0 to 8.5, but it mustn’t vary too far outside that range or even too quickly within it because their blood chemistry cannot cope with either extreme variations or rapid variations.

In either of those cases the oxygen/carbon dioxide transport mechanism becomes very inefficient which could even lead to death.

Where the blood pH and the pH of other organs and cells in the body become too low for normal metabolic processes to continue, the condition is known as acidosis.  When they become too high the condition is known as alkalosis.  In either of these conditions the cause is almost invariably a seriously incorrect pH and death can ensue very rapidly.

The effects of an incorrect pH can be deadly but pH can easily be controlled without using acids or alkalis to correct it. Next month I will describe how. Until then, don’t forget to check your pH.


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