Fish Rx: What Is Ich?

Posted by TF Oren

Ichthyophthirius multifiliis, known also as freshwater ich, freshwater ick, or freshwater white spot disease, is one of the most common diseases suffered by freshwater fish.

Caused by an ectoparasite (a parasite that lives outside the host, either on the skin or as a growth on the skin), ich manifests as tiny white spots or nodules on the fish's body, gills, and fins. Each individual spot, which can grow to one millimeter in size, is a separate parasite called a trophozoite or trophont (a mature trophozoite).

Ich is among the most persistent diseases in freshwater fish because it is so prevalent, so easily introduced into the environment, and so difficult to control once present. A fast-growing parasite, it is almost certainly fatal if left untreated. Ich is not strictly a freshwater disease; there is also a marine version of ich caused by a different organism.


Ich wreaks havoc on the gills and the skin. Contact with tissue is how ich causes the most damage. The parasites cause irritation and can lead to open wounds on the skin, which reduces a fish's ability to regulate the flow of water into its body. Ich also attacks the gills, causing respiratory distress and preventing the fish from getting enough oxygen from the water.

Although treatment can be difficult, fish that survive an infection can develop partial immunity.

In addition to the characteristic white spots on the gills, skin, and fins, symptoms of ich include loss of appetite and/or refusal of all food, respiratory difficulty, cloudy eyes, hiding, resting on the bottom, rubbing or scratching against objects, and swimming upside down near the surface.

Some fish may have subclinical ich, meaning that they will not show symptoms until something triggers their onset. The parasite that causes ich does not have a dormant stage in its lifecycle, so it is always a threat.

Factors or changes in environment that reduce immunity, such as stress, crowding, fluctuating water temperature or quality, poor diet, or the introduction of new fish or other organisms into the environment can trigger an outbreak of ich in a subclinically infected fish already in the tank, or introduce the disease into a healthy tank via an infected organism.


Preventive measures go a long way toward reducing the likelihood of spreading ich. It is important not to buy fish from a tank where any fish are dead or showing signs of disease, and to quarantine new fish for several weeks and new plants for several days before introducing them into an existing tank.

Once an outbreak has occurred, however, there are multiple treatment options. There are commercial treatments available for ich, as well as other methods involving heat, chlorine, salt, and other chemicals. Success rates vary depending upon the nature of the outbreak, how early it is discovered, and how widespread it is. The earlier the disease is discovered, the better the odds that it can be successfully treated.

Interested in learning more about how to keep your freshwater friends healthy and happy? Read more about ich, including preventive measures and treatments, here.

Images by James Pickett and Betta Info/

oembed rumble video here

recommended for you

Fish Rx: What Is Ich?