Fungal Diseases of Fish (Freshwater & Saltwater)

Fungal diseases are a common problem in fish, and can cause serious illness and death. There are many different types of fungi that can infect fish, and they can cause a variety of symptoms.

Some of the most common fungal diseases of fish include saprolegnia, ichthyosporidium, and mycobacteriosis. These diseases can be difficult to treat, and often require lengthy and expensive courses of medication.

Aquatic fungi can cause serious diseases in fish, affecting their skin, gills, fins and internal organs. Some of the most common fungal pathogens include Saprolegnia, Achlya, Pythium and Phytophthora.

These organisms can cause extensive damage to fish tissues, leading to death in severe cases.

The introduction should explain the following:

  • What is a fungus?
  • What are some of the diseases caused by fungi that can affect fish?
  • What are the symptoms of these diseases?
  • How are these diseases treated?

A fungus is a type of organism that can cause diseases in fish. Some of the diseases caused by fungi that can affect fish include saprolegniasis, mycobacteriosis, and chytridiomycosis.

Cotton Wool Fungus (Saprolegniasis)

Saprolegniasis is a fungal disease of fish and fish eggs most commonly caused by the Saprolegnia species called “water molds.”

The disease is characterized by the growth of cotton-wool like fungus on the skin and fins of fish, and on the surface of fish eggs.

Saprolegniasis can cause significant losses in both hatchery and wild fish populations. The disease is most often seen in cool, shallow water where the water temperature ranges from 50 to 86 degrees Fahrenheit.

Causes of Saprolegniasis

Saprolegnia is a genus of ascomycetous fungi commonly found in water environments. Saprolegnia ferax, the most common species in this genus, is the cotton wool fungus.

It is an opportunistic pathogen that can cause infection in both plants and animals. Infection by S. ferax results in a cotton-like growth on the surface of the host.

The fungus can also infect wounds, leading to tissue necrosis. Although saprolegniasis usually does not cause death in healthy individuals, it can be fatal to immunocompromised patients or those with compromised skin integrity. Treatment typically involves antifungal medications and aggressive wound care.


There are several signs that can indicate a fish has saprolegniasis, including lesions on the skin, abnormal swimming behavior, loss of appetite, and presence of the fungus on the body.

When observing tufts of white-colored organic material that form on skin, fins, gills, and other body parts, saprolegniasis is usually the first identification.

Early diagnosis and treatment of saprolegniasis is critical for preventing fish loss.

This fungal infection can cause extensive damage to fish, leading to death in severe cases.