Koi in Hot Weather: When Oxygen Deficiency in the Pond Becomes an Emergency

The current high temperatures are not only taking their toll on us. In a Koi pond, every additional degree can significantly change the conditions as well. And this often happens long before the Koi are visibly gasping for air.

Over the past few days, we have dealt with several emergencies at customers' ponds, all showing a very similar picture: the Koi were lethargic just below the water surface, barely moving and continuously pumping water through their gills. In one case, the situation was already critical. The first Koi were beginning to lose their balance.

In both ponds, the cause was related to the oxygen level.

That is exactly why we want to take a closer look at this subject in light of the current conditions. At water temperatures around 30 °C, aeration somewhere in the filter may simply no longer be enough.

The basic problem is pure physics: the warmer the water becomes, the less oxygen it can hold.

At 20 °C, air-saturated fresh water can contain around 9.1 mg of oxygen per litre under normal conditions. At 30 °C, this falls to only around 7.5 mg/l.

That is a significant difference.

A Koi pond, however, is not a laboratory container filled with clean water. The Koi consume oxygen. The filter biology requires oxygen. Bacterial decomposition processes consume oxygen. Uneaten food, waste and other organic loads also play a role.

The higher the fish stocking level and the more heavily the Koi are fed, the more oxygen the entire system requires.

This is exactly where the problem lies during extreme heat:

The water can hold less oxygen while the Koi and biological processes simultaneously require large amounts of it.

A Koi suffering from oxygen deficiency does not necessarily immediately hang at the surface gasping for air. During hot weather, pay particular attention to the behaviour of the entire group. Are the Koi unusually still just below the water surface? Are they gathering near pond returns or other areas with strong water movement? Are their gill covers moving noticeably faster than usual? It is particularly noticeable when the Koi are continuously pumping water through their gills. Whenever possible, they may also try to rest or lie down somewhere. This is exactly what we observed in the recent emergency cases. If a single Koi is breathing abnormally, gill problems, parasites or other causes may also be responsible. However, if practically the entire group shows similar behaviour at the same time and the water is very warm, the oxygen level should be one of the first things you check.

If Koi begin to lose their balance or roll onto their side, the situation is critical.

Aeration must then be massively increased immediately. Not the next day and not after hours of troubleshooting.

If no aeration equipment is available, fresh cold water can be added to the pond. However, you should avoid cooling the pond by more than 5 to 6 °C. A sudden temperature change is also a stress factor for Koi. Depending on the size of the pond, cooling can happen very quickly or take an extremely long time.

One sentence we hear frequently is:

“My filter is aerated.”

That is fundamentally a good thing. Moving biological filter media in particular requires a great deal of oxygen, and strong aeration in the biological filter is important. However, an aerated filter chamber does not automatically mean that there is enough oxygen throughout the entire Koi pond. The pond contains a large volume of water. The distribution of oxygen depends on water circulation, pond shape, depth and flow conditions. A strongly aerated biological chamber primarily supplies the filter biology working there with oxygen and naturally also enriches the water passing through it. However, it does not guarantee that oxygen is optimally distributed throughout the entire pond. For this reason, we clearly recommend additional aeration directly in the Koi pond.

Because what happens if the filter or circulation pump fails? In that situation, aeration in the filter is of little use because no oxygen-rich water is being returned to the pond.

Aeration is not simply about having a few air bubbles rising somewhere in the pond. A powerful column of air creates water movement. The rising mixture of air and water draws water up from deeper areas and transports it towards the surface. This is where the important gas exchange with the surrounding air takes place. Good aeration can therefore move a surprisingly large volume of water.

This is precisely why we like to use long Japanese air diffuser bars in Koi ponds. A 75 cm air diffuser bar , for example, creates a wide and even column of bubbles. When positioned correctly, it moves considerably more water than a small, concentrated air stone.

However, it is also important that the diffuser is supplied with enough air.

For larger pond aeration systems, we generally recommend using a 16 mm air system. The reason is simple: we want to transport a larger volume of air from the air pump to the diffusers with as little loss as possible. With long, narrow air hoses, pressure losses quickly become noticeable. This is particularly true when several larger diffusers need to be supplied. It can also significantly shorten the service life of the air pump and its diaphragms. We therefore transport the air through a generously sized hose to a suitable air manifold for 16 mm air hose.

From there, the individual diffusers can be supplied and, where necessary, regulated separately. This provides a clean installation and allows the air volume to be distributed properly. A more powerful air pump should not simply be heavily restricted at all of its outlets.

The pump must be able to discharge the volume of air it produces. Constant high back pressure is not ideal for either the performance or the service life of the pump.

As a simple combination that has proven effective in many Koi ponds, we use a 75 cm Japanese air diffuser bar together with a Hiblow HP 40 , for example. The HP 40 delivers 40 litres of air per minute. For a single 75 cm Japanese diffuser, this is a powerful combination capable of creating a strong air column and significant water movement. The air diffuser bar should, however, be positioned sensibly in the pond. Look at the existing water flow. Where are the pond returns? Are there areas with poor water movement? Where do the Koi gather during extreme heat?

Aeration is also part of the pond's overall hydraulics. It should not simply be installed where it is least visible, but where it does not interfere with the existing flow or, ideally, supports it.

If you want to use two 75 cm air diffuser bars, we accordingly recommend a Hiblow HP 80 . The HP 80 delivers 80 litres of air per minute. Using a suitable 16 mm manifold, the air can be distributed between both diffusers and individually regulated. This is useful, for example, in long ponds or ponds with several areas that have different flow conditions. The pump should always be matched to the number and size of the diffusers. A large air pump provides little benefit if the connected diffusers cannot discharge the volume of air being supplied. Conversely, a long air diffuser bar will not perform optimally if it is operated with an air pump that is far too small.

Many pond owners mainly observe their Koi during their free time and therefore do not always notice everything that happens. When it comes to oxygen, however, you should also keep the night and particularly the early morning hours in mind. Algae and aquatic plants produce oxygen through photosynthesis when exposed to light. At night, this oxygen production stops. Processes that consume oxygen, however, continue. The oxygen level can therefore fall during the night. A pond that appears completely normal in the evening can have significantly poorer oxygen levels by early morning. For this reason, pond aeration should not only operate during the hottest hours of the day.

Aeration should run continuously, 24 hours a day.

Another important point is feeding. Koi often continue to show a strong appetite in warm water. However, that does not automatically mean that you should continue feeding them normally. The Koi does not know how much oxygen reserve is left in your pond. At water temperatures around 30 °C or above, we recommend significantly reducing the amount of food. Less food reduces the strain on the Koi and at the same time lowers the organic load on the entire system. There is less waste and fewer substances that need to be biologically broken down. These decomposition processes also consume oxygen. Continuing to feed heavily stocked ponds at 30 °C in the same way as at 24 °C can place even more pressure on a system that is already heavily stressed.

In an acute oxygen deficiency situation, the Koi should not be fed at all.

The oxygen level must first be stabilised and the cause identified.

We also frequently hear the following sentence:

“The pond has been running like this for ten years.”

That may well be true. However, a Koi pond is not a static system. The Koi grow. A few kilograms of fish can become 30, 50 or even more kilograms of fish over the years. The amount of food added to the pond increases and so does the biological load. Then a few exceptionally hot days arrive and suddenly an oxygen supply that appeared perfectly adequate for years is no longer enough. The pond can operate without any obvious problems for a long time.

But once the balance between oxygen supply and oxygen consumption shifts, the Koi can develop clear symptoms within a very short period of time.

Oxygen deficiency is a stress factor and places unnecessary strain on the Koi. It can also make the fish more susceptible to other diseases.

Do not only observe your Koi when feeding them.

How are they positioned in the water?

Are they all breathing calmly?

Are their gill covers moving noticeably faster?

Are the Koi gathering near a pond return or in areas with strong water movement?

Check the water temperature with a thermometer and do not rely solely on how warm the weather feels. At temperatures around 30 °C, you should significantly reduce feeding and take aeration far more seriously than at moderate water temperatures.

And most importantly:

Aerate directly in the pond, not only in the filter. Both should be aerated continuously.

A powerful air pump, generously sized air lines and suitable diffusers are a very small investment compared with the value of a Koi collection. After the emergency cases we have seen over the past few days, we therefore want to say this very clearly once again:

During this heat, do not wait until the first Koi start searching for oxygen at the surface or are already beginning to lose their balance.

If your Koi are lethargic just below the water surface and continuously pumping water through their gills, take this behaviour seriously.

Oxygen deficiency requires immediate action.