How to identify fungal infections in aquarium fish and understand their causes: an explanation.

Fungal infections in aquarium fish typically arise when their immune system is compromised due to illness or injury. In such cases, the fungus exploits this vulnerability and begins to thrive. Pet MD explains how to identify a fungal infection in aquarium fish.

Fungi affect the surface of the skin and scales, without penetrating deeper. Most commonly, infections in fish are caused by water molds (fungal organisms belonging to the class Oomycota). They are found in aquatic environments and usually pose no threat to healthy fish.

Freshwater aquarium fish are more susceptible to fungal infections, as most water molds do not survive in saltwater. However, there are a few species that can thrive in such environments.

Though rare, there are very aggressive fungi, such as branchiomycosis, which can cause debilitating diseases in domestic fish, primarily affecting the gills.

If you notice any signs indicating a potential fungal infection in your fish, consult a veterinarian.

Types of Fungal Infections in Fish

Gill or Mouth Rot

The typical gill or mouth rot in fish is caused by the highly aggressive fungus Branchiomyces spp. This fungus can be a primary cause of fish disease, but it is relatively rare among domestic fish species. Most external fungal infections are water molds, where the fish's body appears mottled or spotted.

Egg Fungus

Opportunistic water mold can attach to various dead tissues, including skin, scales, and unfertilized eggs. Fish eggs that have not been treated with formalin or hydrogen peroxide, or that do not hatch quickly, can easily be subjected to secondary fungal infections from the environment.

Systemic Fungi

While systemic fungal infections are rare, they can quickly lead to the death of domestic fish. Systemic fungal infections are more common in warm water and in aquariums with a higher fish density. Clinical signs may include rapid breathing, sudden death, lethargy, or loss of appetite.

How to Recognize Fungal Infection in Fish

1. White, brown, or fluffy patches on the body, fins, or eyes.

2. Change in gill color, their paleness.

3. Rapid breathing.

4. Loss of appetite.

5. Sudden death.

Causes of Fungal Infections in Fish

Fungal infections most often occur due to other stress factors, including:

• Poor water quality;
• Stress from aggressive tank mates;
• Injuries;
• Inadequate aquarium cleaning;
• Injured or elderly fish;
• Overcrowding in the aquarium.