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Ichthyophthirius Multifiliis (Ich)


Ichthyophthirius multifiliis (commonly known as white spot disease, ich, or ick) is a common disease of fresh water fish. Ich is caused by the protozoa Ichtyopthirius. Ich is one of the most common and persistent diseases. The protozoan is an ectoparasite. White nodules that look like white grains of salt or sugar of up to 1 mm appear on the body, fins and gills. Each white spot is an encysted parasite.

Resources: Wikipedia, the free encyclopedia.

Ichthyophthirius Multifiliis translates to "fish louse with many children". Ichthyophthirius Multifiliis is disease which will be encountered by most Koi keepers, and is easily dealt with if identified early.

Ich is temperature dependent and infections are only likely to occur when the water temperature is below 82° F (28° C). The Ich parasites will attack a Koi by breaking through the top layer of skin (epidermis) and gills, and then attaching themselves to the Koi, where they feed on blood and skin tissue by continually moving in a circular manner.

The fish feels as if it's been bitten by a mosquito. It's not unusual to see infected fish scratching against rocks and gravel in an effort to get relief.

After several days of feasting, the stuffed parasite develops into a trophozoite, tunnels out of the fish and sinks to the bottom of the pond. Secreting a soft jellylike substance, it forms a protective membrane inside of which it divides into hundreds of baby parasites, known as tomites. The hungry tomites soon leave their home in search of a fresh fish to dine upon.

Typical signs of Ich include:

  • Anorexia (loss of appetite, refusing all food, with consequential wasting)
  • Rapid breathing
  • Hiding abnormally/ not schooling
  • Resting on the bottom
  • Flashing
  • Rubbing and scratching against objects

In advanced stages you can see it with the naked eye. It looks like little white spots that eat away at the base of the fins and cluster around the gill openings. In early stages it takes a microscope to see these circles that move round and round. Inside the circle is a nucleus shaped like a horse shoe. We do not want to wait for Ich to get so bad that it can be seen without the microscope. Ich prefers killing goldfish to the bulkier koi, but still a threat to koi as well.

The disease is highly contagious and spreads rapidly from one fish to another. It can be particularly severe when fish are crowded. While many protozoans reproduce by simple division, a single "Ich" organism can multiply into hundreds of new parasites. This organism is an obligate parasite which means that it cannot survive unless live fish are present. It is capable of causing massive mortality within a short time. An outbreak of "Ich" is an emergency situation which requires immediate treatment: if left untreated, this disease may result in 100% mortality.

The ich protozoa goes though the following life stages:

  • Feeding stage: The ich trophozoite (a protozoan in active stage of life) feeds in a blemish formed in the skin or gill epithelium.
  • After it feeds within the skin or gills, the ich falls off and enters an encapsulated dividing stage (tomont). The tomont adheres to plants, nets, gravel or other ornamental objects in your pond.
  • The tomont divides up to 10 times by binary fission, producing infective theronts, thus dividing rapidly and attacking the fishes.

This life cycle is highly dependent on water temperature, and the entire life cycle takes from approximately 7 days at 77 °F (25 °C) to 8 weeks at 43 °F (6 °C).

"Ich" is the largest known parasitic protozoan found on fishes. Adult organisms are oval to round and measure 0.5 to 1.0 mm in size. The adult is uniformly ciliated and contains a horseshoe-shaped nucleus which can be seen in older individuals.

The breeding stage of the parasite encysts between the layers of the host skin. When mature, it leaves the fish and produces large numbers of free swimming young. These must find a host within 48 hours (at water temperatures of 75-79°F) or they will die.


Prevention of Ich is preferable to treating the fish after a disease outbreak is in progress. All incoming fish should be quarantined for at least three days when temperatures are 75 to 83°F. At cooler temperatures a 3-day quarantine will be inadequate for Ich because of its lengthened life cycle. For this reason, and to prevent introduction of other diseases which have incubation periods greater than 3 days, a longer quarantine is strongly recommended. Three weeks is generally considered a minimum period for adequate quarantine of new fish.

Ich will take advantage of situations in which Koi become stressed and their immune response becomes weakened, so good husbandry and water management will help to keep the occurrence of this disease to a minimum. Ich is also triggered by water temperature change and is a common problem after new Koi are added to a pond, as the water temperature is usually different and they will be stressed from transportation.

Changes in water temperature are a major contributor to the occurrence of ich so the installation of a heating system for your pond will help in maintaining a stable water temperature. Ich may also be introduced on plants and live food, so always disinfect these before adding them to the pond. This can be done by dipping the roots of plants and immersing live food in a potassium permanganate bath of 0.8g per 4.5 liters (1 gallon) for five minutes. However, this may have a detrimental effect on more delicate plant species, so it is better to avoid adding plants and live food if possible.


When ich has reached its mature stage and it is visible, it is relatively immune to treatment as it is embedded between the top two layers of skin. So when treating ich you must aim to kill the free-swimming stages. Standard dosages of formalin/malachite solution or copper sulfate will kill the free-swimming tomite stage, but the trophont and tomont stagesa re resistant to treatments. This is why treatment must continue long enough for all the trophonts to develop into tomonts and then into new tomites, when they are susceptible to treatments. Salt in the pond at 0.3% (3g NaCl/liter water) will inhibit Ich infestations.

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