Why Are My Koi Carp Jumping Out Of My Pond?

Andrew HodgesHealth, Water QualityLeave a Comment

Artistic vector illustration of two Koi carps playing in pond. Hieroglyph means Koi.

After long hours spent designing and building your beautiful new koi pond, filling it with sparkling clear fresh water, you are ready to sit back and enjoy your hard work.

But rather than worry free evenings sitting by the pond, your fish are jumping out and committing suicide. If you have come home to fish dried up on the outside of the pond, you are probably wondering what on earth has gone wrong.

As a naive new fishkeeper some twenty years ago, this was my experience. I have learned a lot since then. Let me help you diagnose the problem and more importantly get things back on track.

So why do koi carp or other pond fish throw themselves out of the water?

Well, I have divided this issue into four main categories:

  1. Quality of original water source. Was the original water used to fill or top up the pond treated or filtered to deal with chlorine, chloramine metals etc.?
  2. Pond filtration efficiency and maturity. Has the filter system matured enough to deal with the biological load in the pond, i.e. fish waste? Is the water quality within safe limits for koi health? Has the filter been maintained properly?
  3. Fish health. Are the fish healthy and free from disease or are the suffering from a parasite, virus or another infectious disease? Have you recently added new fish?
  4. Natural behaviour. Are the fish trying to catch flies, is it the mating season, are the fish simply exploring their environment?

In this post I will give as much information as I can on these topics, I am not a vet. I am expressing my views from experience as a koi carp lover.

Source Water – water is just water right?

Water is essential to all living beings; it is an extraordinary substance. Your tap water will have a unique composition — dependant on the characteristics of the watercourse.

From one area water may be heavy in salts and minerals. Water from another area which has passed through different rocks and soils on its journey to you may have a completely different composition.

Beautiful koi carp pond and bonsai tree

In an area where there is granite rock, water will remain acidic and soft, as it passes through the impermeable rock.

Where there is limestone or chalk prevalent in the area, calcium salts will make the water alkaline and hard.

The mineral sediment through which the water passes may add traces of heavy metals such as aluminium, iron, lead, mercury, cadmium, zinc or bismuth!

Agricultural areas may lead to nitrates, used in fertilisers finding their way into the watercourse. Although considered safe for human consumption high nitrate levels are toxic for Koi Carp.

So in a word, no water is not water, there can be a vast difference in its composition, just not apparent to the human eye.

Water treatment is good for us, not so good for aquatic life.

Since the turn of the twentieth-century Chlorine has been added to our water to irradicate water-bourne diseases.

Chlorine in combination with ammonia forms a range of substances known as chloramines, which stay in the water for longer than chlorine. This treatment is useful for ensuring that drinking water remains safe to drink, but can be lethal to our fish.

Aerial view of storage tanks in sewage water treatment plant

Chloramine and Chlorine must be removed from your tap/source water before you add fish. Toxicity is detectable in the gills at relatively low chlorine concentrations. At higher levels, it can burn fishes gills and cause significant distress.

If chlorine and chloramine are left, fish may jump from your pond water to escape the distress.

A specialist pond water filter should be used to fill and top up your pond. If you do not have a water filter or your pond is already full, water conditioning treatment. Treatment must be used at an appropriate concentration, to remove chlorine and chloramine as soon as possible.

Japanese breeders use mountain spring water

As seen water companies may add many and various chemicals to our water, some of which can be harmful to our koi. But the Japanese breeders do not have this problem because they get their water from the ice melting on the mountains and flowing down into their mud ponds.

For their mud ponds, Japanese breeders do take advantage of the freely available mountain spring water. Their indoor ponds, however, are often filled with treated mains water. Breeders have to filter in the same way as a hobbyist. All be it their systems are commercial in scale.

Interestingly in both situations, the koi breeders maintain a constant trickle of fresh water into their ponds. In the indoor ponds, this is usually at a rate of around 5 to 10 per cent of the total water volume per day.

Hotaka mountain range and azusa river in spring at kamikochi national park nagano japan

Japanese koi carp breeders stress that it is VITAL to filter mains water before it enters any koi pond.

From time to time water companies will dose the supply with chemicals designed to kill insects and larvae or to clean out pipework. Or they may raise usual dosing for a specific reason.

So it can be seen that even if you have not filtered your water in the past and never previously experienced an issue, you may just have been lucky.

Current levels of chlorine in a pond can be measured accurately using DPD chlorine test tablets available online. DPD tablets can also be used to check whether your filter cartridges are still efficiently removing chlorine.

Is your pond filter working efficiently – both mechanically and biologically?

To maintain perfect water in your pond, we need to address the pollution within it. Your pond is a closed system and therefore requires constant monitoring.

One important point to note is that the primary pollutant is the food we put into our ponds, not the fish. Stop or reduce feeding if you are experiencing issues with water quality.

If you are confident that your original water was filtered or treated appropriately with water conditioner, the next step is to test your water.

There are several key test koi keepers should undertake every week:

  • Ammonia
  • Nitrite
  • Nitrate
  • Oxygen
  • PH
  • Water hardness
  • Phosphate

Test kits are available at koi dealers or online, and it is worth spending your money on a comprehensive test kit.

Follow the instructions carefully and record test results week to week.

Testing for Ammonia

Fish excrete Ammonia via their gills and urea as dilute urine. This waste is known as nitrogenous waste.

Urine is broken down within the pond by microbial activity into ammonia.

One of the most toxic substances to koi carp found in your pond is Ammonia, like chlorine, ammonia will burn the fish’s gills.

Koi Carp will sometimes leap out of water having a high concentration of ammonia to escape the burning or irritation.

If your pond is brand new, your filter may not have matured or developed enough beneficial bacteria to deal with the ammonia excreted by your fish.

Where your pond has been established for some time, filter bugs may be low on oxygen or low in numbers. Alternatively, the filter may require maintenance, cleaning and flushing through.

Where your water test indicates high ammonia the following advice would be sensible:

  • Using a filter start product to increase beneficial bacteria;
  • Add air stones in filter bays or where water enters the filter;
  • Make constant water changes to dilute the ammonia (filter or condition).
  • Perform appropriate filter maintenance. Ensure there is no waste build up inside the filters.
  • Add a proprietary koi pond clay to your water. Clay adds minerals required for the healthy development of beneficial bacteria and fish.
Koi carp in clear water
8570442 – koi carps swimming in a water garden

Testing for Nitrite

In the filter system, the ammonia is converted by nitrification into nitrite by beneficial bacteria called Nitrosomonas. Nitrite is slightly less toxic than ammonia but is still toxic to fish.

If your water test indicates high levels of nitrite, this would suggest that the filter is not effective in this stage. The advice here is as for high ammonia levels, mentioned above.

Testing for Nitrate

The next stage of the nitrification cycle is the conversion of nitrite into nitrate. Nitrospera, another beneficial bacteria, converts the nitrite into nitrate.

Nitrate is less toxic to fish than nitrite. If nitrates are allowed to build to a high concentration, it can become a serious issue. Your test kit will show you acceptable and unacceptable levels.

When your water test indicates high nitrate levels, your filter is efficiently dealing with ammonia.

The steps taken to lower nitrate levels include:

  • Test your incoming water to ensure that it is not adding nitrate to your pond;
  • Use a filter to remove nitrates from incoming water;
  • Constant trickle water change using filtered water;
  • Use an anoxic filter to remove nitrates efficiently (see here);

Testing for oxygen

Low oxygen levels in the water can cause distress to fish, as they are suffocating. Low levels of dissolved oxygen could be a cause of koi carp jumping from the pond.

More commonly fish will be seen gasping at the water surface where oxygen levels are low.

Typicaly in the warmer months of the year oxygen levels will be lower.

This problem is easily remedied using an air pump and air stones or agitating the water with a pump or fountain which will entrain oxygen.

The many other water tests available

I shall cover the other water tests in a separate article. This article is concerned with koi carp jumping out of the pond water. So I won’t go too deep into the full spectrum of water tests.

Follow the instructions within your test kit.

Fish health, are your fish ill?

In my experience, the first thought when fish act strange, flick or jump is that they must have a disease. You begin to question the ethics of your koi dealer.

Fish health is often not the cause of the stress. We must go through all of the above water quality issues before looking to treat the pond with chemicals.

I would recommend taking a sample of your pond water to your koi dealer to get a second opinion on the water quality. If everything is ok, then you will be in the correct place to discuss your concerns.

If your fish do have some form of parasite, virus or disease, then follow expert advice. Please, before treating your pond with any of the harsh chemicals, all too easily purchased at koi shops.

A tip an old koi enthusiast gave me a long time ago:

“don’t anticipate problems with your fish!”

Natural behaviour – fish being fish will jump

The final section of this post concerns the fact that fish will naturally explore their environment. Koi carp are intelligent creatures and want to test the boundaries of their environment.


They will jump out of the water playing, chasing, escaping predators such as cats or heron. Fish will also behave erratically during mating season, jumping and bashing into one another.

Koi Carp love to eat natural foods such as flies and bugs, so they could be jumping to catch these.

Unfortunately, the fish may accidentally land on the outside of the pond, for no real reason. Finding your treasured koi carp dried up on the grass is heartbreaking.

Covering the pond with netting or building some form of side fence or barrier may stop the accidental exit.

Another consideration is that herons and cats will sometimes take fish out of a pond. Herons may spear a fish which it later finds to be too big to swallow, so leaves it on the ground!

So in summary – why do your fish jump out of your pond?

  • They are not happy with the water quality;
  • They have a virus, parasite or another disease;
  • They are behaving as fish do – naturally.

But don’t jump straight to the worst-case scenario – illness!

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