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.

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