Fish Fungus 
© ademyc / stock.adobe.com
Fish fungus is caused by fungi that naturally occur in the water and can infect damaged fish species.
Fish fungus belongs to the group of fungal infections in fish. However, there’s usually more to it than just the pathogens. Learn more in this article about how fish fungus develops and what you can do to tackle it.
Table of contents
- How dangerous is fish fungus?
- Which fish are particularly susceptible to fish fungus?
- Symptoms: What are the signs of fish fungus?
- Diagnosis: How is the disease detected?
- Treatment: How are infected fish treated?
- Causes: What triggers fish fungus?
- Infection: How does transmission occur?
- Prognosis: What are the healing chances with fish fungus?
- Prevention: How to protect your fish from fish fungus
How dangerous is fish fungus?
Fish fungus (Saprolegnia) primarily attacks aquarium fish that are already weakened. This fish disease is also referred to as water mould. The extent to which the fungus harms your fish depends on a variety of factors.
Severe infestations can be fatal for your fish. Consequently, preventative measures are indispensable in the fight against fish fungus.
Which fish are particularly susceptible to fish fungus?
In general, any fish can be affected by fish fungus. While some ornamental fish such as catfish might be a bit more resistant, other species are more sensitive. This includes, in particular, fish with long fins.
Injuries in these slender areas are especially prone to quick colonisation by the pathogens.
Is fish fungus contagious to humans?
Humans cannot contract fish fungus, even with contact with infected animals. Nevertheless, please be aware that you can still transfer the pathogens.
Always adhere to general hygiene practices when dealing with aquariums and water bodies, which are further explained in the prevention section.
Symptoms: What are the signs of fish fungus?
As the fungal infection progresses, it increasingly spreads across the fish’s body. Initially, small white spots form at entry points such as eyes, fin edges, mouth edges, or nostrils.
Gradually, the characteristic cotton-like deposits form at these white spots. Moreover, serious damage to fins can occur: they fray, stick together, and eventually dissolve.
The consequences are often life-threatening for the affected fish.
When should I see a vet?
If you notice the first signs of fish fungus in your fish, you should act swiftly. The sooner you consult a knowledgeable vet, the greater the chance of recovery, and the more likely you are to spare your aquarium dwellers further suffering.
© dsom / stock.adobe.com
Diagnosis: How is the disease detected?
The external characteristics of fish fungus can quickly be identified by vets familiar with fish diseases. To confirm suspicions, the vet will take a sample of the white deposits and examine it under a microscope.
Under bright light, the pathogens show up as long and transparent threads (hyphae).
Treatment: How are infected fish treated?
To combat the fungal infestation, you should optimise the living conditions in your aquarium. Increase the water temperature or transfer your fish to new or clean old water.
Depending on the size of your fish, the vet or you yourself can gently remove the white deposits with absorbent cotton.
For severe fungal infestations, you can use special agents against fish fungus in consultation with your vet. This includes commercially available products or potassium permanganate, which you can add at a concentration of 0.5 grams per ten litres of water.
Causes: What triggers fish fungus?
Fish fungus is a fungal infection where fish contract the oomycete Saprolegnia. The most notable representatives are Saprolegnia parasitica, Achlya, or Aphanomyces.
These fungi naturally occur in water and infect the skin and eggs of many fish species. However, a prerequisite is that the fish’s skin is already damaged. Thus, fish fungus mainly occurs when a fish has a bite or puncture wound.
Environmental conditions, like water temperature, can also trigger fish fungus. One of the most common causes is hypothermia.
Infection: How does transmission occur?
Saprolegnia reproduce mainly through small spores that enter the water through human or natural influences. As they possess motile appendages called flagella, they can move in water.
Once they reach further developmental stages, they can anchor themselves to the damaged fish skin and form hyphae. These grow into the surrounding tissue and cause life-threatening damage.
The pathogens can easily enter your aquarium through new arrivals or contaminated plants and objects. In open waters, the transmission often occurs via contaminated fishing equipment.
© lapis2380 / stock.adobe.com
Prognosis: What are the healing chances with fish fungus?
If fish fungus is advanced and you don’t provide your fish with timely medical assistance, the prognosis is usually grim.
For this reason, it is particularly important to take preventative measures to prevent the onset of the disease.
Prevention: How to protect your fish from fish fungus
Success in treating fish fungus isn’t always achievable. Thus, prevention is the best means to safeguard your fish from infection. To achieve this, you should heed the following advice:
- Regularly observe the behaviour and appearance of your fish to identify changes promptly.
- Place new arrivals and new plants into a quarantine tank before introducing them to your aquarium.
- Check the water hardness of your tank regularly and adjust it as required.
- Periodically carry out a water change.
- Ensure the stocking density is not too high to prevent stress and fights among your tank’s inhabitants.
This article from zooplus Magazine is purely informative. If your pet is experiencing discomfort, contact your veterinarian for diagnosis and treatment.
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At the Justus-Liebig-University of Giessen I was extensively trained as a veterinarian and was able to gain experience in various fields such as small animal, large animal and exotic animal medicine as well as pharmacology, pathology and food hygiene. Since then, I have been working not only as a veterinary author, but also on my scientifically driven dissertation. My goal is to better protect animals from pathogenic bacterial organisms in the future. Besides my veterinary knowledge, I also share my own experiences as a happy dog owner and can thus understand and enlighten fears and problems as well as other important questions about animal health.
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