What are the ideal water conditions for healthy koi?

There will certainly be a lot of differing opinions on the best water conditions for koi but one thing remains constant: poor water quality leads to a host of other health problems that are certainly avoidable with proper care.  Because stress originates from poor water quality, follow these guidelines for healthy water and healthy koi.

Dissolved Oxygen

Oxygen levels should be at a minimum of 5.0 mg/L for koi.  In the fish world, there is some variation with the tolerable level of dissolved oxygen in the water but 5.0 is a good baseline.  As a reference, 5.0 is the minimum for koi and 18 mg/L is the physical maximum that water can hold.  Just as a reminder though, cold water can hold more dissolved oxygen than warmer water so higher summer temperatures (and overcrowded ponds) will lead to lower dissolved oxygen, which is really when you will want to pay attention to it the most.  Oxygen becomes dissolved into your koi pond in several ways.  At the very surface there is limited diffusion occurring between the atmosphere and the water and that accounts for only a small amount of dissolved oxygen.  Turbulence will also agitate the water enough to generate dissolved oxygen in your pond and this often comes in the form of falling water as from a small water fall.  A tried and true and popular way to increase the dissolved oxygen in a koi pond is by an air stone on the bottom of the pond or by spout or fountain shooting water up into the air.


Okay, back to chemistry class everyone because its time to discuss pH.  As you may recall pH has something to do with acids.  Its all about acidity and alkalinity.  The pH scale is a logarithmic one meaning when your pond jumps from 7 (neutral) to 6, its not simply getting a little bit more acidic, its getting 10 times more acidic.  Conversely, when your pond tests at 7.5 and then gets increasingly alkaline by jumping to 9.5,  then your pond just got 100 times more alkaline (10 times 10).  So it’s a big deal when your pH changes and can definitely affect your koi in negative ways.  pH, or power of Hydrogen, should range in your pond somewhere between 6.8 and 8.2 but do your best to keep it as stable as possible.

Ammonia, Nitrites, Nitrates

As you may recall, the general cycle of waste in your pond starts with ammonia excreted by your fish then bacteria and oxygen break it down to nitrites which later get broken into nitrates then free nitrogen.  Ammonia, nitrites and nitrates all have the ability to cause health problems for your koi if their levels are not kept in check.  Along with other health problems, ammonia can essentially burn your fish’s gills and reduce its ability to extract dissolved oxygen from the water.  High nitrites can damage your koi’s kidneys and nervous system and high nitrates, for extended periods, can cause your fish’s immune system to be compromised.  Ammonia and nitrites and the most troublesome when it comes to health problems for your koi but don’t underestimate the power of nitrates over long periods of time.  Here is a guideline for these three water chemistry measurements.


ammonia: levels should be zero. Depending on your pH, you can get away with 0.5ppm (parts per million) or 1 ppm for a short period of time but keep in mind that above a pH of 8.0 ammonia becomes more toxic.


nitrites should be less than 0.25 ppm but ideally you should have a reading of zero.


nitrates: a reading of 20 to 60 ppm is acceptable.



The use of salt in koi ponds has, for long time, been a tried and true method to deal with various water quality and health problems that arise.  Some of the benefits of salt is that its a cheap way to keep some disease at bay, control algae and may also lower nitrite toxicity. Additionally, salt plays a part in the osmotic pressure between the fish and the outside aquatic environment.  There is a differential between the solute concentration of the fish’s blood and the fresh water that it swims in so the addition of salt actually lowers that concentration differential and makes it easier on the fish by reducing the amount of work its body has to do.  A salinity of up to 5 ppt (parts per thousand) or 0.5 % is acceptable.


Temperature obviously plays a big role in the overall health of your pond and  it warrants your attention.  Temperature can exacerbate existing problems, especially higher temperatures.  For example, warmer water holds less dissolved oxygen and ammonia can be more toxic.   Although koi can handle temperatures of between 35 and 85 F degrees its best to keep your fish in water that ranges between 65 and 75 F degrees.  And as with pH, try to avoid large temperature swings.

20 Responses to What are the ideal water conditions for healthy koi?

  1. donna haddock says:

    We have just installed a koi pond and currently have two koi. We have lost three. One of the remaining Koi has some thread-like trailers from his gills. There are no lesions on his body, but he is very lethargic (so is the other Koi). The koi with the trailers eats, but very little. I’ve never seen the other koi eat at the surface. The water tests are fine. We have been adding additional amounts of beneficial bacteria to build the ecosystem of the pond. We are so new at this that we don’t know what to do. I suspect, though, that this is fungal. We love the fish and the pond. Please give us your expert advice as soon as possible. Thanks!

    • Koi-Care staff says:

      Yes, it sounds like cotton wool disease. Here is an excerpt from my site about it:

      Cotton Wool Disease

      Another disease Koi sometimes suffer is called Columnaris, which is sometimes also called Cotton Wool Disease. White threads in the fish’s mouth and a dry skin appearance are the main characteristics of this disease. Sometimes the color of the Koi becomes darker and white sores can appear on its skin. It is not always easy however to determine if the fish has the fungus in its gills. However, if the Koi stays near the surface of the pond, gulping for air, it is a good indication that the fungus is, in fact, in the gills. The Koi can also develop a soggy belly and a slimy coating over its skin.

      Treatment: You can put potassium permanganate in the water to help clear up the condition. Additionally, injecting antibiotics and treating the wound directly will help care for the disease. You should separate the infected Koi from the population and treat the water so the other fish do not become infected.
      Good luck

  2. Margaret Circle says:

    How much is to much water movement

    • Koi-Care staff says:

      Well, these fish aren’t really built to swim against huge currents like a salmon would be so you want enough water movement to move oxygenated water around the pond in a more subtle way. You don’t necessarily want to create a situation where they are swimming against a current.

  3. lincoln yates says:

    We have moved into a new house with a pond three weeks ago and have killed three coy carp already they look fine when when you look at the deadfish. What are we doing wrong

    • Koi-Care staff says:

      could be a lot going on there. Think back and analyze what you have done to/with the pond so far. What kind of food have you given them? Have you put city water into the pond? Is your pump still working? Obviously, whatever the previous owners did worked and something you are doing is different from that routine. Usually the culprit is water chemistry -you may want to check that. For information on what your water chemistry should look like see my page here (http://koi-care.com/ideal-water-conditions-healthy-koi/)

  4. We have had 9 koi for over ten years without any problems. Recently we have had a large blue heron remove 3 koi (right next to the plastic decoy) and had problems with our auto water fill. One month later after the heron and three months after the auto fill problems the six remaining koi got ill with flukes and other diseases which I am having a difficult time identifying and eradicating. Have these diseases been caused because of the heron or water problems? Any suggestions?

    • Koi-Care staff says:

      Water quality issues are usually at the root of most koi and pond health problems so that is always where I advise people to start. The heron could have certainly caused some stress but that really would have been temporary (stress can cause your fish to be more prone to disease). Most likely you are having an issue with water quality that is allowing things like harmful bacteria or fungi to thrive and affect your fish. High ammonia can be common and that usually leads to high nitrites & nitrates. If you think you have flukes you will need to treat the whole pond -here is an excerpt from my site about the topic:

      There are two main classes of flukes; 1) gill flukes and 2) skin flukes. Both kinds are microscopic so if you suspect your fish are suffering from flukes you will need a microscope to verify it. Dactylogyrus or gill flukes will attach to the gills and, as the name suggests, body flukes or gyrodactylus attach to the body. Having hitch hikers is usually not a problem…except if you’re a koi and they are parasitic! Flukes tend to eat away at the protective “slime” coating of your fish thereby exposing them to harmful bacterial infections and causing ulcers. They also are an irritant and cause your fish to itch and subsequently rub themselves on the bottom or side of the pond in an attempt to itch and rid themselves of the parasite. Gill flukes can eat away at your koi’s gills so much that the koi will not be able to assimilate oxygen from the water. If you suspect a fluke infection be sure to look out for this behavior.

      Treatment: You can quarantine but it is likely that your entire pond is afflicted with the flukes or their eggs so you can treat with medications like Aqua Prazzi, potassium permanganate or Fluke Solve (both AquaPrazzi and Fluke Solve have the same active ingredient; “praziquantel”).

      Another thing you can do is regular water changes (not too drastic) and you will also want to boost your beneficial bacteria by using something called Microbe-lift PL. If you have deaths in the future you can always take the carcass to someone that specializes in fish or perhaps a nearby university where they can look under a microscope for certain parasites. You may also want to think about cleaning out the built-up muck at the bottom of your pond if you don’t do that on a regular basis. How do these dead fish look when you pull them out? ulcers? raised scales? anything obvious that would indicate what caused their death?

  5. Martin Clarke says:

    I have built a pond 16ft x 8ft x 4.5ft two years ago . I have had disease after disease loosing many of my koi carp. My most common disease are sores. Each time I test the water quality or take my water samples to my local aquatic centre I/they always find that the water quality is perfect. I have also took the aquatic s advice on treatments but find that all the treatments I have use seem not to work. Can anyone out there (carp expert )advise me. Or better still come and view my pond

    • Koi-Care staff says:

      Well, sores are signs of bacterial infections and bacteria are always going to be present in any pond. If the fish are already compromised in some way then the existing bacteria will move in and affect your fish and cause ulcers and sores. Is it possilble that you have animals trying to get at your fish and stressing them out or perhaps injuring them and causing the injuries to become ulcerous? I don’t know what treatments were recommended to you but if it was the wrong treatment for the problem then no good would come of it. For example, Mela-Fix is an antibacterial/antimicrobial and that doesn’t mean its going to do anything for viruses. My suggestion is start by bringing the salt concentration up to 0.5% for a few days as well as using the appropriate dosage of Mela-Fix. You will need to do water changes after that to reduce the salt level but do it gradually.
      Another thing you may want to think about is seepage of pesticides or other unwanted chemicals from sources that may not be obvious. Maybe the ground that you built your pond in has something leeching out of the soil that doesn’t show up on water tests that are only looking for ammonia, nitrites etc.?

  6. kevin r. says:

    Helo, I pulled up your site on google, I have a fresh water tank koi, he is about 5 inches long, and he a a problem with his mouth. It is awollen and redish pink, he is not eating and he goes to the top for a gulp of air accasionally and he just lays on the bottom of the tank. Please help me figure out what wrong with him and what to do, he is in the tank with two small sword tail goldfish and one large goldfish, i have them in a 40 gal tank, thank you very much.

    • Koi-Care staff says:

      Mouth rot is a symptom of something larger and that is usually poor water quality that is causing your fish’s health to be compromised and open to infection. You’re going to want to stop feeding for the time being and start improving your water quality by doing a 30% water change. Next thing would be a light salt bath treatment and perhaps a treatment of Mela-Fix. You will also want to monitor your water quality by testing it regularly. You can also remove the fish and treat the sores with hydrogen peroxide or iodine.

  7. Edna Ramirez says:

    We moved into a home with a koi pond, but had no fish. We got 3 small koi and have had them for 3 months now, they loved to eat ( they are growing fast ), swim around and they have never been social with us. They seem scared. Anyhow about 3 weeks ago they started hiding under the waterfall, never come out anymore during the day. They do swim around at night. They also don’t eat much. The temp has changed. It was a very hot summer and now its much cooler especially at night? Git any suggestions. They don’t look diseased.

    • Koi-Care staff says:

      One thing that koi in an unprotected outdoor pond may have issues with is predators such as large wading birds. They may be trying to get at your koi and thereby scaring the koi, making them not want to come out. Not eating as much is associated with falling water temps and slower metabolisms. See my latest infographic on my site for feeding charts based on temp.

  8. Roman says:

    I have a 3 koi/3 goldfish 2 pleco tank in my kitchen, they are doing wonderful in there water is clear. We also have a few small real plants. One of the koi is a white/ silver color and one goldfish is white/ orange and black. My question is, why do they have a green tint devoloping on the white parts only? They do nip the greens often and are fed “API Goldfish Pellets” which say they “promote digestion and color” the greenish color on the gold fish is primarily the mouth and fins. (both fish tints starting from the body out to the mid fin area, and from the mouth to the between the eyes)

    Thank you

    • Koi-Care staff says:

      I can’t say that I’ve ever come across that before. Usually if you have color modifications occur they become more muted. Green tints are usually associated with algal growth.

  9. Brandon Chang says:

    Hi Everyone,I am new in keeping kois.Plz can you
    suggest me the ideal water parameters in keeping kois.
    My water out here has a TDS of below 100,PH-7.5.Can kois be
    kept in these conditions.

  10. Lisa says:

    Hi. Please advise me as to which plants or reeds are acceptable to the koi invoiroument. My mate has so many intruders in his pond. The reeds have taken over. There is not much sun. The depth and width, Im not too sure of, but it seems large and ideal. He had 45 koi of note. Stunning colors, large in size, massive energy, jumping out the water, so very spectacular in every way. John lost them all as he had a four day power failure. He was given eight lovely little ones. He was also given many gold fish. They swim together in the reeds, the debris from huge trees, such as, yellow stink wood, syringas, jacarandas plus a host of other alien creepers.

    I would dearly lve your input as we love our few little ones.

    Thank you so much for your advice.


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