Koi pond oxygen

Koi breathe by pumping water over their gills to extract essential oxygen, which is invisibly dissolved in the pond water.

Fortunately, compared to other fish species, koi do not require an especially high level of dissolved oxygen (DO) which makes them relatively undemanding in this area. Even when, through whatever reason, DO does fall below a koi’s minimum requirement, they can adapt by coming to the surface to gulp in air as a short-term survival strategy. However, this is extremely stressful for fish and if you are witness to such an extreme change in behaviour the alarm bells should start ringing.

As is the case when confronted by any pond or koi-related problem, a responsible koi keeper should approach it in 2 ways and ask a couple of questions.

How can I remedy the problem and avoid stressing the fish any further?

What can I learn from this experience as far as what caused it and how can I prevent it from happening again?

A gasping response in fish will not necessarily always be caused by an unhealthy drop in DO but may also be behaviour shown by fish that is not getting sufficient oxygen to its tissues for another reason.

Besides a drop in DO (which is the most likely cause of gasping behaviour) another cause could be that the koi are suffering from poor water quality, specifically nitrite toxicity.

If for some reason, toxic nitrite is not being broken down and detoxified effectively by a biofilter, then it will cause a build up of nitrite within a koi’s blood and tissues. When nitrite reacts with the oxygen-carrying haemoglobin within the blood, it forms a stable non-oxygen carrying form of haemoglobin, which will in turn cause the fish to gasp.

For this reason, in addition to adding extra aeration it would be prudent to carry out a water test to identify if the cause of the problem is associated with a water quality problem.

Other more common causes of gasping in fish can usually quite simply be identified and easily rectified. These include:

Hot Weather:

Yes, occasionally we can suffer from hot weather in the UK. As water warms up, its ability to hold DO decreases and yet a fish’s oxygen requirements increase. The DO requirements of other organisms in the pond such as bacteria and other aquatic creepy crawlies also increase.

If a pond is running close to a comfortable oxygen level prior to such a hot spell then when the water temperature rises, the ‘double whammy’ affect described above can leave koi and the pond environment oxygen-deficient.

A well-planted pond in full sunlight is a real asset as a natural source of oxygen as the plants produce far more oxygen in photosynthesis than they use in respiration. However, at night, the same plants will still be using oxygen and will not be releasing any oxygen through photosynthesis. This will cause a drop in pond DO causing the fish to gasp first thing in the morning as a result of a phenomenon called dawn depletion. A remedy for this is to thin out excessive submerged plant growth and improve aeration or circulation within the pond.


Some pond medications when added to the water can cause quite a severe drop in DO through chemical reaction. Medication may also reduce DO through adverse interaction with other pond life. Extra aeration should be added during any course of treatment or medication as a precaution and extra care should be taken after the use of an algicide. Dead algal matter will put an extra burden on the oxygen budget within the pond as it is broken down by bacteria.

Blocked filter/pump.

This cause of low DO may sound obvious but a blocked pump or filter may cause problems by stealth where a gradual, almost unnoticeable drop in turnover occurs. This will have a knock-on adverse effect on DO levels.

Regular flushing and maintenance of filter media and pump pre-filters will keep this from becoming a real problem. Blockages are more likely to be an issue in the warmer months when excessive algae growth is experienced.

It is reassuring to know that a pond can not be over-aerated and through the many different means and devices available today, barring accidents, low DO should not be a problem in a well managed koi pond.

Filtration will nearly always benefit from the addition of extra aeration which in turn will lead to better water quality. Venturis, air stones, micro-bubblers, and even air domes fitted to bottom drains allow easy, effective and efficient aeration and are a must for any serious koi keeper.

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Kill blanketweed and string algae.