Is Fish Fungus Contagious?

Have you noticed that your fish has developed a fungal infection? Do you also have other fish in your aquarium that you’re worried about? Is fish fungus contagious?

Evidence shows that fish are susceptible to getting fungus from other infected fish.

Don’t worry though, most fungal infections are easy to treat.

Is Fish Fungus Contagious?

There seems to be some debate on whether fungal infections in fish are truly contagious. But, fish experts say fungal infections are not generally contagious.

Still, healthy fish kept in the same aquarium as an affected fish are more likely to develop an infection.

It’s always best to be cautious and quarantine your infected fish for treatment. Doing so seems to keep the rest of the tank safer and less likely to develop a fungal infection.

What Is A Fungal Infection?

A fungal infection appears as white, cottony threads that pass through organic matter. Fungus feeds on organic matter, breaking it down with specialized digestive juices. We call the threads “hyphae”, and you’ll notice them sprouting from your fish’s body.

The most common areas where fungus develops include the mouth, gills, and fins. Still, fungi can develop anywhere on the body, particularly once the infection worsens.

Fungal infections are usually treatable as long as you catch them early. Because they grow on the gills and fins, it can be hard for your fish to swim and breathe. So, if you let the infection go for too long, it can be fatal.

In aquariums, Achyla and Saprolegnia are the kinds of fungi most often present. They are water molds found in almost every aquarium. They do not usually cause a problem because a healthy fish’s immune system can fight them off.

Why Might A Whole Aquarium Become Infected?

Fungal infections are not usually contagious. So, why would an entire tank of fish develop an infection?

Fungi are opportunistic and will take root anywhere they’re able to. Fungi is present in almost all aquariums, but they don’t affect healthy fish. They will only begin to attack a fish once it’s become sick.

The most common causes of a fungal infection are dirty tank water, cold water, and a lack of oxygen.

A Dirty Tank

If you don’t clean your tank properly, waste will continue to build up. This leads to a spike in ammonia, nitrites, and nitrates which are toxic to fish.

Your fish’s immune system becomes compromised. They will become more susceptible to disease and illness, including fungi.

Cold Water Or A Lack Of Oxygen

Water that is too cold has a similar effect. If you’re housing tropical fish, then you need to keep their water warm. If the temperature drops too much, their immune system becomes compromised.

Similarly, too little oxygen in the aquarium leads to a compromised immune system.

Underlying Sickness

Plus, fungal infections are usually secondary conditions pointing to a larger problem. Your fish might have a bacterial or parasitic infection. If one fish is sick, it’s likely that your other fish are sick, too.

This can lead to an entire aquarium developing a fungal infection in no time.

What Should You Do If Your Fish Becomes Infected?

The first thing you should do is separate your fish from its tankmates. Place them in a quarantine tank where you can treat them alone.

Treat Your Fish

You should add aquarium salt to the tank. Aquarium salt is wonderful for treating wounds and diseases.

You’ll likely need an antifungal medication as well. Medications like Maracyn, ParaGuard, and SulfaPlex are excellent for treating fungal infections.

Find The Cause Of The Infection

Then, you need to find out what’s caused the infection in the tank. Test your water to make sure the ammonia, nitrite, and nitrate levels are in check.

Check your heater and thermometer to ensure the temperature is correct. Also, make sure your filter is producing enough oxygen, and buy an airstone if need be.

If everything with the aquarium is okay, then you’ll want to watch your fish for signs of illness. Parasites and bacterial infections are common issues that lead to fungal infections.


Whether fungal infections are truly contagious is up for debate. But, fish in the same aquarium with an infected fish are more likely to catch that fungus. Quarantine the affected fish to prevent your entire tank from getting the infection.

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