Fungal Diseases in Freshwater Fish

Fungal Diseases in Freshwater Fish
By Asst. Prof. Dr. Somphoth Weerakul (DVM)


Diagnosis and Characteristics
When a fish has a wound, we often observe white to brown cotton-like tufts floating in the water or attached to the body. Fungi are opportunistic pathogens that strike when a fish is weak or injured.

It is crucial for keepers to distinguish fungi from certain protozoans that share this "cotton wool" appearance. A simple test is to lift the fish to the water's surface:

Fungi: The tufts will collapse and flatten against the body.
Protozoa: The structure remains unchanged.
Identifying the pathogen correctly is essential for choosing the right treatment.

Common Fungal Species
The most frequent culprits in freshwater fish are Saprolegnia spp., Aphanomyces spp., and Achlya spp. Most fungal infections are superficial. However, Aphanomyces is more dangerous as its hyphae (fungal filaments) can penetrate deep into the muscle tissue, causing severe ulcerative wounds, often accompanied by secondary bacterial infections.

Infection occurs easily via floating spores or hyphae in the water looking for a food source—your fish's wounds. The danger level depends on the extent of the infection; widespread cases lead to a loss of electrolytes, which can be fatal.


Treatment Options
Treating fungal infections is difficult once they have spread extensively. Effective options include:

Malachite Green: Highly effective but strictly prohibited for food fish as it is a human carcinogen.
Salt: Can be used, but the concentration required to kill fungi is much higher than that used for bacteria. It is generally used in ponds to inhibit growth.
Formalin: Often used in ponds but is ineffective against certain fungal strains.
Modern Alternatives: Newer specialized treatments include Bronopol, Eugenol, and Thymoquinone.

Dosage and Administration
1. Malachite Green (Best Choice)
Standard Bath: Use a low concentration of $0.1\text{ mg/L}$. Immerse for a full day, repeating every 3 days for a total of 3 treatments.
Egg Treatment: To treat infected fish eggs, use $1\text{ mg/L}$ for 1 hour.
Direct Application (Best for localized wounds): Prepare a solution of $100\text{ mg/L}$. Dip a cotton swab and apply directly to the wound once daily for at least 5 days.
Note: Becomes toxic in soft water or at high temperatures. It is also toxic to aquatic plants.
2. Salt
Use a concentration higher than $3\text{ g/L}$. Maintain the bath all day. Perform partial water changes and replenish the salt daily until the wound improves.
3. Formalin
Mainly used for disinfecting eggs; however, it can be fatal to fry once they hatch.
Dosage: $1\text{ ml per 38 L}$ of water. Immerse for 12–24 hours.
Procedure: Change 30–70% of the water and re-dose daily until symptoms improve.
Note: Watch for pale skin or abnormal breathing, as it becomes toxic in soft water or high temperatures.
4. Bronopol
Use $15\text{–}50\text{ mg/L}$ for a 30–60 minute bath. Higher concentrations are required for treating eggs.
Crucial Note: Fungal treatment must be performed alongside treatment for the primary bacterial or viral infection that caused the initial wound.

Prevention is Better Than Cure
If you notice a fish has an injury, immediately add salt to the water. Salt helps maintain electrolyte balance and reduces stress while inhibiting fungal growth. Consistent water changes and maintaining a clean environment are the best ways to keep fungal problems at bay.