Ichthyophthirius multifilis under a microscope

Short for Ichthyophthirius multifilis, Ich is another freshwater parasite that loves to hide in the skin of your living jewels. The ciliated protozoans are recognizable under a microscope with the U- or horseshoe-shaped nucleus in a round body.

Ich feed off dead skin cells and blood. The sand-grain sized white, waxy spots on the body and fins that gives the disease its informal handle—White Spot Disease—are the encysted parasite at an advanced stage.

A few words about Ich and shipping fish

  • The risk for an Ich infection is at its highest during shipping, and when fish are first added to your quarantine tank or pond.
  • Ich is always in the water. Under normal conditions, fish have no problem keeping Ich at bay. However when they are stressed and their immune systems weakened, there is a chance for infection.
  • If you receive new fish and do nothing to prevent an Ich infection, the chances are good your fish will be affected, particularly with small fish (6″ and smaller).
  • A salinity level of .35% (or 3.5 parts per thousand) is all that is needed to prevent an Ich infection.

Although the spots themselves aren’t fatal, the secondary infections that can result from the parasite leaving the fish can be. It is the damage caused to the gills that often results in death. Smaller and younger fish infected with Ich have a higher mortality rate.

Ich is probably the most widespread of all diseases affecting koi, and many keepers will come across it. Ich frequently occurs when water parameters aren’t set, or the pond hasn’t had time to settle in and in cooler temperatures.

Because a koi with encysted Ich (known as a theront) sometimes will not exhibit any symptoms associated with Ich, it can easily introduce the parasite unnoticed into a pond that is Ich free.

Once the theront is mature, it drops off and falls to pond or tank bottom. It then divides rapidly and becomes a very mobile form (known as a tromite). These fast-swimming parasites attach to and then reinfect any other koi in your pond.

Take a look at our video of Ich under a microscope

Water Temperature

Water temperature, as it often does, plays a big part in the life cycle of Ich. The cooler the water, the longer the life cycle. The norm is 2-5 days; however, with cooler water it can be upwards of 5 weeks.

Although warmer water temps over 85ºF can stop Ich, heat-resistant strains of Ich are now emerging. The same is true of salt resistance.


Unfortunately, the parasite can live on your koi without causing any obvious symptoms associated with the full blown illness. However, these are some of the things that can indicate you have Ich in your population:

  • flashing (rubbing against the sides or bottom of the pond bottom),
  • lethargy
  • appetite loss
  • small white spots – it will appear as if someone has sprinkled grains of salt onto the fins/body of the fish


As the parasite burrows into the skin cells, it is very difficult to kill until it is mature and drops off the body. Once it is out “swimming” in the open it is easily destroyed.

A salt solution of 0.3%-0.5% at no more than 80ºF is the most recommended treatment; however, if your koi are already in distress, 0.5% might be a little too high of a salinity.

The recommendation from koivet.com is to bring the proportions to the correct levels incrementally over a few days. This helps to slowly acclimate your fish and your filter without damaging them.

Depending on the water temperature, the salinity should be maintained for up to 21 days to eradicate the parasite in a pond environment.

The problem is that in colder temperatures, the parasite will encyst for longer periods, making it more difficult to gauge how long to leave the salt in the pond or tank. As a guideline, however, at 50ºF it should be anywhere from 14-21 days, at 60ºF from 10-12 days, 65-70ºF from 7-8 days and at 75-80ºF 2-5 days.

A few words of caution, though, when using salt:

  • It can lower the freezing point of pond water in regions with colder temperatures.
  • It can also kill your water plants. Removing those plants that you don’t want to lose is advisable.
  • Make sure there are no additives that can prove harmful.

If salt fails, malachite green and formalin are the next go-to lines of defense. However, at lower temperatures they might not be as effective. Make sure to read the instructions.

If you suspect an Ich problem, cleaning the pond’s gravel is a great way to get rid of the dividing Ich parasites before they can repeat the cycle, as they replicate on the bottom of your pond or tank. Gradual water changes of 30-40% will help to recalibrate the salinity in your pond.

We understand how stressful treating koi illness can be. It is why we are fastidious about the quarantine process that all of our koi fish for sale undergo before we make them available. Additionally, we have a koi health resource page to cross reference symptoms and treatments if you think that your koi are ill.

Contact us to see how we can help you get you your next living jewel.

11 responses

  1. Ray Kelsey :

    I have a 350 gallon pond, and my koi show the signs of ich. I cannot raise the temp of the water. Can you recommend a treatment or a product to treat?

    1. rosaline :

      Sorry to hear you are having problems with Ich. If you are not able to heat the water, you can treat with salt for a longer period of time. As a guideline, at 50ºF it should be anywhere from 14-21 days, at 60ºF from 10-12 days, 65-70ºF from 7-8 days and at 75-80ºF 2-5 days. If a salt treatment fails, you can try Malachite Green or Formalin. At lower temperatures, these might not be as effective as the salt, so you will want to carefully read the provided instructions on the bottle. Hope this information helps!

  2. Hello! I have a 4800 gallon pond and in all the years of having a pond with koi, I’ve never had a problem with ich until this year. I have approximately 10 koi, 4 have ich spots. I live in Wisconsin and the water temp is about 56 degrees. I did a 50% water change before treating with Eco Labs BSDT32 Broad Spectrum Disease Treatment and then a 25% water change before a second application a couple of days later. The koi are all doing well, eating and actively swimming. How long does it take for the white patches to disappear? Do I need to treat again? Thank you in advance!

    1. Hey Deb,

      We are sorry to hear that some of your Koi have been affected by Ich this year. The most ideal treat would be what is mentioned in the article about adding pond salt to your water at a solution of 0.3%-0.5% and a raised water temperature at no more than 80ºF being most recommended. However, since you have used medication, you will need to check if salt can be used in conjunction with it or continue to follow the instructions on the packaging for usage until no visible signs of Ich remain.

      We hope that your fish will recover soon. Thanks!

  3. Michael Armando :

    Hi I have a 2400 gallon fish pond. I had this pond for 30 years. Most of the years I had the pond I never had a health issue with the fish. I have Koi and Goldfish which are more than 12 years old. I noticed the other day they were acting strange. Staying on top at night which they really never do. I noticed some swimming fast and flashing along the bottom. I checked my water PH and everything else and the water was fine. I even brought my water to a fish lab and they said the water was healthy for fish. I have a lot of air in the pond with two waterfalls also so the fish do get a lot of air. The water temperature right now in the pond is 76 degrees. It would be hard to get it up to 80 degrees unless we have some really prolonged hot days. I started adding pond salt slowly over the last two days. I added so far 23 lbs. I have to add much more. I had the water checked for salt. It came back 500 PPM which equals 0.05%. In your article you say to get it to 0.3 to 0.5 for treatment. So as you can see I am not even at 0.1% yet. I will add more salt each day until I reach 0.3 to 0.5%. How long should I leave the salt at that level once I reach that level. They say more than 0.1 for Koi fish is not good. Is that true? Would this treatment help? Thanks.

    1. Hello Michael,

      We are very sorry to hear that you have been experiencing some issues with your pond recently. As far as the salt level is concerned, we maintain a .35% salinity in all of our holding tank at our facility. At that percentage, the fish will build a thicker slime coat to protect themselves from naturally occurring bacteria and parasites that can be found in most water sources. You can leave salt at that level as long as you like, but not recommended to keep it any higher unless you were treating an illness for a period of time.

      If you have any other questions, send an email to sales@nextdaykoi.com.

      Thank you.


    Hello,my koi have had a ruff summer. We were attacked by racoon and we lost 20, 18 yr old fish.I miss my beautiful big koi so much,I cryed for days. I recently added some small koi and they all diyed . They were from a friend and I didnt seperate them, what a mistake. Now the few fish left are swimming fast and hitting them selves on the bottom of the pond.I dont know how much water Is in the pond to gage the amount of salt to add,or what kind of salt to get. Please help me as Im so distrought about my beautiful pond being a mess. Ive never had any problems up untile the coons came around,we caught them and fenced and electrifyed the out side of the pond,now the fish are displaying signs of trouble. Please help 503 829-6874

    1. We are very sorry to hear about the predators that got to your fish. From your description of the remaining fish, the flashing behavior you are seeing is generally associated with a parasitic infection or beginning signs of parasite bother the fish. If you have the means of quarantining the fish in a fish tank or set up large enough to temporarily hold them, you can better gauge the amount of salt to use. Raised salinity and water temperature would be best to treat for Ich. We maintain a .35% salinity in our tanks. This reduces stress and aids in promoting natural healing. Bringing the water temperature between 75-80 degrees will speed up the recovery process when coupled with the salt usage. You can purchase salt from most Lowes Home Improvement or Home Depot stores sold as a water softener. A popular brand available is called Solar naturals.

  5. Michael lr :

    Hi, can nox itch be use with salt? The salinity level in my pond is at 0.5 %

    1. Yes, you can treat Ich with pond salt. We maintain a .35% salinity in all tanks in our facility, as it helps them build a thicker slime coat to protect themselves from bacteria and parasites like Ich found naturally in water sources.

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