Saprolegnia in Freshwater Fish

Causes, Treatment, and Prevention

Tetra with Fuzz on Skin

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Saprolegnia (also known as water mold or cotton mold) poses a serious threat to fish in freshwater aquariums. These aquatic fungal species create fuzzy patches on a fish's skin or gills that resemble tufts of cotton. Healthy fish are often able to ward off infection, but Saprolegnia will readily take advantage of sick or injured fish, promoting secondary diseases. Untreated, Saprolegnia infections can be deadly.

What Is Saprolegnia?

Saprolegnia is a genus of several opportunistic fungal pathogen species found in freshwater aquariums. These water molds feed on dead organic materials and replicate by forming spores that float around freely, waiting for a bit of dead tissue to inhabit and grow into. Dead skin cells on fish with injuries or weakened immune systems make ideal hosts.

Symptoms of Saprolegnia in Freshwater Fish

Saprolegnia is easy to identify by its one telltale symptom:


  • Fuzzy patches on skin or gills

Typically, the fuzzy patches of Saprolegnia are white or gray, but if your tank's water contains a lot of algae or other debris, the patches may be green, red, or brown.

Damage to the skin or gills exposes the fish to secondary attacks from bacteria and other fungi, which can quickly lead to death.

Causes of Saprolegnia

While Saprolegnia may be present in your aquarium water at any given time and not cause problems among your healthy fish, there are predisposing factors that increase the likelihood of infection. These include:

  • Poor Water Quality: Fish swimming in contaminated water (from algae and debris) are more vulnerable to many illnesses. The chronic stress caused by trying to maintain homeostasis in poor-quality water leads to decreased immune function, making it much easier for bacteria, other fungi, and parasites to take hold and cause disease.
  • Inappropriate Temperature: A tropical tank that is kept too cool or experiences a sudden drop in temperature can cause an increased incidence of Saprolegnia infections.
  • Immunosuppressed Fish: Some fish do not have the robust immune capabilities of their counterparts and are more susceptible to different diseases. By noting individual fish differences in disease susceptibility, you can catch early signs of disease by monitoring "weaker" fish. These fish with compromised immune systems may be more susceptible to opportunistic pathogens such as Saprolegnia.

Diagnosing Saprolegnia in Freshwater Fish

Saprolegnia must be diagnosed by an aquatic veterinarian before treatment. Water mold can look very similar to Columnaris bacteria but requires different treatment.

Your vet will do a skin mucus scrape or swab culture from a live fish to determine the problematic pathogen. Dead fish cannot be used to make a diagnosis because Saprolegnia growing on the dead tissue may mask the presence of Columnaris.


Saprolegnia is not responsive to antibiotics, but secondary bacterial infections require them. You may need to treat both bacteria and Saprolegnia fungi, using different medications.

Your veterinarian may prescribe injectable medication or medicated foods to target bacterial infections

(this will limit the effect of antibiotic medications on the health of your (biologic filtration).

Due to its opportunistic nature, most Saprolegnia infections will heal on their own when you fix the underlying cause (temperature or water quality) and treat bacterial infections. Occasionally, though, stubborn cases of Saprolegnia may require the addition of antifungal medication to the water.

Prognosis for Freshwater Fish with Saprolegnia

Severely sick or immuno-compromised fish often die from Saprolegnia because the mold spores cause an aggressive and swift-moving infection. Mildly affected fish should recover well with prompt treatment and improved environmental conditions.

How to Prevent Saprolegnia

The best method of preventing Saprolegnia is to maintain your water quality and feed your fish a good diet to promote their natural immune function. Keep up with your maintenance regimen, keep debris vacuumed out of the gravel, and have a hospital tank ready to quarantine any potentially sick individuals.

In addition, check to make sure your heater is functioning well at all times. A malfunctioning heater can cause temperature irregularity and potentially drastic drops in temperature that can encourage Saprolegnia.