Pond fish koi are gasping?

Dissolved oxygen concentration             

Q: At various times this summer I have seen my Koi gasping at the pond surface and around my waterfall. Does this mean that there is not enough oxygen in the pond? Is this because of exceptionally high temperatures or for some other reason? I have a waterfall and a Blagdon air pump with two airstones; my pond is 3,500 gallons. Do I need more aeration than this? What can I do to prevent this happening in the future? If this is due to a lack of oxygen, will this do any damage to the health of my Koi?

A: Thanks for your letter which details your koi-watching observations, and provides your own helpful suggestions as to the cause of your gasping koi. Understandably, you may be surprised by an apparent DO (dissolved oxygen) deficit as you are already adding supplementary oxygen via aeration and a waterfall. What I find even more surprising is that you have experienced sufficiently hot weather this summer to cause a low DO problem in the first place! Let’s explore….

Does this mean that there is not enough oxygen?

Before being able to confirm whether your pond is sufficiently aerated during hot weather, it is first worth considering other factors that could lead to similar gasping behaviour in your koi.

1. Nitrite: Besides a drop in DO (which is the most likely cause of the gasping behaviour you have observed) another cause could be that your 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 your biofilter, then it will cause a build up of nitrite in your pond water and then subsequently within your 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 your koi to gasp.

For this reason, the next time you observe your koi gasping (I hope you don’t once you’ve read this reply), it would be prudent to carry out a water test to eliminate nitrite as the cause.

2. Medication

Another factor to investigate and eliminate is pond medication. 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 as a precaution and particular 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. Have you been using any medications periodically that may have preceded any gasping incidents? If not, then you can eliminate medications as a possible cause.

Is this because of exceptionally high temperatures or for some other reason?

Hot Weather: Your low DO prime suspect.

Oxygen behaves in a surprising way (compared to other solutes, such as salt) in that as the water temperature rises, its ability to dissolve oxygen reduces. Compare this to salt (which dissolves better in warm water) and you can start to appreciate the unusual relationship water has with oxygen. So as water warms up, its ability to hold DO decreases and yet your koi’s oxygen requirements increase. The DO requirements of other organisms in your pond such as bacteria and other organisms will also increase.

If your pond had been running close to a limiting oxygen level prior to each hot spell then when the water temperature rose, the ‘double whammy’ effect of both pond organisms demanding more oxygen and your pond water holding less DO would have left your koi and pond environment in an oxygen-deficit.

….some other reason

Dawn depletion is another factor besides high temperature can cause DO to drop so low that it will cause your koi to gasp. In a natural planted pond, full sunlight is a real asset as a natural source of oxygen as the plants (and algae) produce far more oxygen via 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.

But as you observe your koi to be gasping in hot weather (mid afternoon), then this is an unlikely cause – but nevertheless, worth keeping an eye on.

Do I need more aeration than I currently have?

If warm weather seems to be tipping your pond’s DO budget over the edge, you need to consider whether your pond is receiving sufficient oxygen to allow the complete array of organisms to respire efficiently, recognising that for your mature pond to remain balanced, all contributory organisms must be able to function healthily. Furthermore, focussing on the koi themselves, your pond is supporting diverse populations of micro organisms (that are in turn, sustaining your koi’s environment) and if oxygen becomes limiting, will lead to a deterioration in your water quality and koi health.

All organisms that respire aerobically require oxygen. This includes heterotrophic bacteria that breakdown organic matter (and autotrophic bacteria that use oxygen during their release of energy from inorganic compounds such as ammonia and nitrite), fungi, protozoan organisms that scavenge the pond for debris and larger metazoan organisms that will filter and graze on organic material. Plants too respire constantly throughout the day and night, which includes, of course blanketweed (if you consider your pond to be unplanted) and any other algae adhered to pond sides and pipework, as well as any larger aquatic plants that may be found within a planted pond. All of these vital organisms rely on a supply of oxygen, just as a burning fire does to keep it functioning effectively. Deny them oxygen, and the water quality will deteriorate; add extra and the pond will roar into life, just as a bonfire does in a gust of wind.

I am a little concerned that the existing arrangement of airstones and a waterfall is not sufficient for your pond. This leads me to question whether the stocking and/or feeding levels are both too high for your pond. It is difficult to give specific advice on how many koi should be kept in a 3500 gallon pond so I would double check by asking other pond and koi keepers to visit your pond and to verify its stocking density.

If having eliminated all other causes of a low DO you are able to conclude that the warm weather is the sole cause, then you need to look at adding extra aeration.

You could try to see if your existing air pump can be used to make the necessary improvements.

Having recognised that your koi are in competition with other pond organisms for oxygen, add a few more airstones to your biochamber. This will provide the extra aeration where it is needed, leaving more available for your koi. Research has shown that the most efficient aeration is achieved from the pond bottom using a fine-bubble diffuser. Try additional air stones at first (if nothing else, to check that your existing air pump can pump down to the pond bottom). If you are not so happy with additional trailing airlines into your pond then I suggest you consider a single airdome (which can be fitted onto a bottom drain, hiding your airline within the waste pipe. Compared to airstones, air domes produce a greater number of finer bubbles.

If this is due to a lack of oxygen, will this do any damage to the health of my koi?

DO levels that are sufficient to cause your koi to gasp will cause you koi problems both directly and indirectly.


Koi will be stressed by both acute and chromic incidences of low DO. The gasping response is a koi’s method of accessing oxygen directly from the atmosphere (because the DO levels cannot supply their need). This is a short term strategy as it cannot be sustained indefinitely as CO2 levels will increase further in the blood and oxygen levels will fall until they become critical to the life of the koi. The usual hormonal response associated with a stress-event will be released, making koi more susceptible to pathogens.


Without oxygen, filter bacteria cannot oxidise (and detoxify) ammonia into nitrite and then into nitrate, making these toxins accumulate in the pond. If a pond or filter are allowed to become oxygen deficient for an extended period of time, anaerobic conditions will prevail causing unstable water conditions and the tell-tale bad-egg smell. This will adversely affect water quality and stability which in turn will jeopardise koi health.

In conclusion, on the face of it, your low DO problem should be easily rectifiable, perhaps even using your existing air pump. Double check that you are not trying to achieve the impossible by overstocking and overfeeding your koi, otherwise your pond will be very vulnerable should your supplementary aeration ever break down.

Kill blanketweed and string algae.