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 mg/L 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.


No discussion of pH is complete without also talking about KH (or carbonate hardness or alkalinity).  Your pond’s KH level will affect how susceptible your pond is to pH fluctuations.  If, for example, your KH reading is particularly low then anything you do to get your pH back to an acceptable range may be short lived as your pond lacks the ability to buffer future pH swings.   KH should be around 105 ppm with a possible deviation of plus or minus 15 ppm (up or down from 105).  Baking soda (sodium bicarbonate or bicarbonate of soda) will increase your pH making it more basic. Conversely, you can add white vinegar to lower, or make more acidic, your pond water.

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.

This entry was posted in Pond Care and tagged ammonia, dissolved oxygen, health, KH, koi, nitrates, nitrites, pH, salt, water chemistry, water temperature. Bookmark the permalink.

34 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.


  11. Lynn says:

    I have my first koi pond which was opened about 2 months ago. I purchased a TDS-3 meter to check the salt level. I have the readings but have no clue what they mean. It is 107 x 10 ppm. Is this bad or good? My master test kit show every I know to measure is good. My pond now gas algae.

    • Koi-Care staff says:

      I’m afraid I’ve not used that testing device before. If you have algae then you most likely have a build up of algae food like ammonia and nitrites. You need to make sure that your filters are doing their job and what helps is seeding them with beneficial bacteria like Microbe-Lift PL. You can also plant some aquatic cover to provide shade and less sun to fuel the algae.

  12. Peter Moua says:

    Help. I’m a newbie to koi keeping. If anyone can help me answer some question, I super duper highly appreciate it. I have a 89g pond with 6 (3inch) koi. My questions are:
    My water quality is great, but koi fish keep flashing. If they’re is chemical toxin in pond, what should I do?
    One of my koi fishhas blurry color skin, what is that? Doesn’t look like infection or fungus. Its skin looks burry only.
    What is the best Pond Bateria product that I should use?
    Why do koi rip each others fin? There is so much room to swim around.
    How do I know when koi are ich and fungus free?

    • Koi-Care staff says:

      hi peter, welcome to the world of koi.
      flashing usually means some sort of external parasite that they are trying to rid themselves of. Go to my article on diseases-you’ll see lots of parasites discussed(http://koi-care.com/koi-diseases-treatments/).
      blurry skin-I’m not familiar with that.
      pond bacteria? microbe-lift PL
      ripped fins could come from abrasions against the side and butterfly koi are prone to fin splitting.
      ich and fungus free: you wont see any white growths on their skin.

  13. Cheryl says:

    My koi are in an aquaponics system in southern UT where we are experiencing triple digit weather. Any suggestions on how to cool the water? We’ve been putting ice blocks in and managing to keep it around 85.

    • Koi-Care staff says:

      Shade and chillers. I’ve never had temps where I had to employ chillers but they should do the trick.

  14. Carolyn Koester says:

    My 3 month old koi pond has green stringey stuff (I guess it’s algae) hanging from rocks/waterfall. The pond is located in a corner where garage meets the house and gets little sun. I need a primer on water testing as this is all new to me. I read in one of your posts some of the readings I should strive for but I haven’t even purchased the water testing kit (brand suggestions?) much less I don’t know what to add to attain optimal levels as suggested. Get me started in the right direction before my koi start having problems like I’ve read about. I live in the mid-west and have raccoons not birds to worry about. Thanks ever so much.

    • Koi-Care staff says:

      Okay, so you’ve got string algae and I cover that in detail here (http://koi-care.com/getting-rid-of-algae/). There’s really only two main types of pond algae that you need to be concerned with and that’s one of them. If you want to know more about the nuances of your water chemistry I also have a good article that explains it in layman’s terms here (http://koi-care.com/understanding-biological-cycle-koi-pond/). You mentioned that you already found my article on the ideal water conditions and as far as test kits there isn’t one that does it all but there is one I can recommend that is easy to use by API and its called “API Pondcare 5-in-1 Pond Test Strips” -you just dip the strip in the water and get the reading. Lastly, if you are just starting out I also recommend a book (around $20) that goes through everything from beginning to end -if you end up not finding it useful you can always get a refund (http://koi-care.com/koi-resources/). Good luck!

  15. ashlee says:

    hello recently i have noticed bumps on my koi carp, so i netted some today and 2 fish have similar symptoms, one fish bumps look white and waxy and others is more pink looking, there is not many bumps on the fish just one or two. im wondering what the best way for me to try and treat the problem? please get back to me asap


  16. ashlee says:

    thanks for the quick reply, i think it is carp pox, do you have any suggestions on how to heat the water? and what temp should it be? i have not ever tested my water and would like to begin, i have a ph tester and its 7ph, what would you suggest as the next test to buy as money constraints mean i cant purchase a full test kit.. ammonia, nitrite or nitrate?? are any more fatal to health than others??
    i look forward to hearing from you and thanks for the help its grately appreciated

    • Koi-Care staff says:

      you don’t want to exceed 85 deg. F and don’t want to raise the temp. more than 10 deg. F in a span of 18 hours. There’s not a whole lot you can do about it, its just around in ponds. You could certainly install a UV Clarifier in your system but they are pricey and if you have money contraints then that won’t be an option. Carp pox will go away on its own. About testing kits, they are all important but ammonia has the potential to do the most damage.

  17. Mike h says:

    Have high ph level. What a good way to bring the high level down to a manageable level.

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