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Fin Rot 101: Treatment & Prevention Tips


Staff member
Jan 19, 2024
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Fin rot is a serious disease that all aquarists need to be aware of. If left unchecked, it will wreak havoc on any affected tank.

But many fish owners don’t know much about it at all!

In fact, we’d say that most owners only start learning about fin rot after they notice it affecting their fish (not before).

So we’re on a mission to change that. This guide covers everything you need to know about fin rot so you can not only treat it, but prevent it from happening in the first place.

What Is It?​

Fin rot is a common disease that can affect all freshwater fish species. As its name would suggest, this disease targets the fins and tail of your fish.

It’s most often associated with fish that have long flowy fins, such as Fancy Goldfish (other types too) and Betta fish.

However, the disease does not discriminate. Symptoms are most identifiable with long-finned fish, but fin rot is equally as harmful to species with short or transparent fins.

A betta fish with fin rot

When fish suffer from fin rot, the flesh of their fins start to slowly disintegrate and fall off. The fins may appear raggedy and discolored. Eventually, the disease will spread to the base of the tail and start affecting the body.

Author Note: There are many ways that the disease can affect fish. However, at its core, it is a bacterial infection.

At first, it might appear to be nothing more than a cosmetic issue. But, the disease has the potential to be fatal if it’s not addressed! Not only that, but fin rot can lead to other infections, fungal problems, and a host of other health issues.

If you suspect that your fish has fin rot, it’s important to take action as soon as possible!

Is Fin Rot Contagious?​

Like many other freshwater fish diseases, fin rot is contagious. The disease is caused by gram-negative bacteria.

This type of bacteria is quite dangerous because the outer membrane is hidden by a slime layer. Thus, bacteria is strong and easy to spread.

Other fish in your aquarium can “catch” the disease through physical contact with the infected fish.

To make things even more challenging, the nature of your aquarium is perfect for spreading contagion. No matter how large your aquarium is, it’s still a closed environment. Bacteria can infect the water and cycle through your filtration system.

This causes the bacteria to quickly spread and infect other fish.

Common Causes​

There are a few causes of fin rot, but the most common is poor water conditions. Fish continually produce waste, which raises the nitrite and ammonia levels throughout the tank.

This slowly sours the water, allowing pH levels to increase and bacteria to flourish.

Author Note: This is especially true in tanks with a lot of decaying materials. Whether it’s leftover food or dying plants, those materials can cause the water quality to go south fast!

When this happens, harmful bacteria can start attacking your fish. The most common types of bacteria that cause fin rot are Aeromonas, Pseudomonas, and Vibrio. All three of those bacterial species are gram-negative, which helps the disease spread.

Stress is another big cause of fin rot. In fact, stress and poor water conditions go hand-in-hand. Fish will become increasingly stressed out the worse water quality gets. However, the disease can take hold even with other stress-inducing issues, too.

Fin rot is also known to affect fish in overcrowded community tanks or environments. Excessive handling and frequent moves can also lead to increased stress levels. Some fish may also suffer from fin rot if the environment has temperatures that are too low for them!

Finally, fish can suffer from fin rot after experiencing physical harm.

Fin-nipping is a pretty common form of aggression that many fish species are known for. Those long and flowing tanks make fish swimming targets!

For the most part, fin-nipping is nothing more than a periodic inconvenience. But when the tank is small and the vulnerable fish is outnumbered, it can become a serious issue. Fin-nipping leads to physical damage to the fins. In turn, the bacteria can take hold and worsen the problem over time.


Symptoms of fin rot are very easy to spot, but the nature of the disease means it gets progressively worse over time. Your fish could suffer from the disease for several days before you start to see major symptoms.

In the early stages of the disease, you might notice a subtle white or semi-transparent edging on the fins. While this symptom is most apparent on the tail fin, the disease can affect all fins. This white edging is subtle. Depending on the appearance of your fish, it could blend in with the natural color.

As the disease progresses, this white edging will get thicker and thicker. The affected area may take on a more sinister color. The infection could stay white, but it could also take on a gray, black, or even brown color.

Eventually, the fins will start to fall apart. This creates a frayed and uneven appearance. Large chunks of flesh will simply slough off, shortening the tail dramatically.

Author Note: In bad cases of fin rot, your fish might experience a secondary infection on those frayed edges. Columnaris, also known as cotton-wool, infections are quite common and affect fish in the midst of a fin rot infection. It’s a fungal issue that makes the flesh look puffy and malformed.

If fin rot is left to attack your fish for too long, the disease can take away their entire fin. At this point, the infection will start attacking the body and the base of the fin can become inflamed.

In addition to all of those physical symptoms, you might notice some behavioral changes in your fish as well.

The disease takes a toll on the fish because their body is working overtime to fight the infection. As a result, fish often become lethargic and weak.

They may avoid fish and spend most of their time resting at the bottom of the tank (even if they aren’t bottom-feeders). Most fish also lose their appetite and start avoiding food.

Fin Rot Treatments​

Fin rot affects the body slowly and progressively gets worse over time. Think of the progressive damage to your fish’s fins as a timer!

This means the quicker you treat the issue, the better. Addressing the disease fast will reduce the severity of the symptoms and make the overall impact on your fish’s body less serious.

The good news is that the disease is relatively easy to treat. There are several over-the-counter medications that you can use.

These treatments are very effective. But, you’ll also need to do a lot more than medicate your fish to truly get rid of the bacteria that cause the disease. Here are some ways to treat fin rot.

1. Quarantine The Infected Fish​

The very first thing you need to do is remove the infected fish from the community tank. Remember, fin rot is a contagious disease.

Removing the affected fish will reduce the spread of the bacteria. Plus, it will allow you to treat the fish without causing harm to others in the community.

Set up a separate quarantine tank that’s appropriately sized for your fish. Make sure to get the water parameters just right before introducing your fish into the new environment. Be gentle when moving them! The last thing you want to do is cause further damage to the fins.

2. Analyze Water Quality​

Once the infected fish is safe in quarantine, take some time to get a better understanding of why the disease took hold at all. The first place to look should be water conditions.

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You can pick up water testing kits at most pet stores. Use them to analyze pH levels and potential contaminants. Your test should be able to give you more information about the ammonia and nitrate levels. While you’re at it, use a thermometer to check the temperature.

All of this information is going to help you prevent fin rot in the future. It’ll also help you make necessary changes now to prevent other fish in the tank from coming down with fin rot as well.

3. Clean The Tank And Make Necessary Changes​

You’d be surprised by how much a deep clean of your tank can help. Cleaning the aquarium might not treat existing fin rot infections. But, it can prevent the spread of the bacteria.

Remove any old food or decaying plant matter. Then, use a substrate vacuum to get rid of any waste.

If your aquarium is in bad shape, you might just want to break the environment down and disinfect everything. Going this route allows you to remove grime from all surfaces and get rid of bacteria once and for all.

Whatever you do, make sure that you’re creating stable water conditions for your fish.

4. Use Antibiotics And Medications​

There are many different medications available to treat fin rot. If you go to your vet, they will likely prescribe a powerful antibiotic to get the job done. A prescribed antibiotic typically works faster. It may also be more effective because your vet can prescribe the right antibiotic for your fish.

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Alternatively, you can use over-the-counter medications. Erythromycin is a popular treatment option that many fish-keepers swear by. Erythromycin is an antibiotic that treats a wide range of infections.

In the fish trade, the antibiotic is marketed as a broad-spectrum treatment that can tackle all kinds of diseases. This includes fin rot! Typically, it comes in powder form. Just follow the instructions on the packaging to provide the appropriate dosage for your fish.

If your fish is experiencing a secondary fungal infection, you can also use medications like methylene blue.

Author Note: For good measure, we recommend removing the carbon filter when you use medications. The carbon filter could remove some of the medication from the water, which you don’t want.

5. Monitor Your Fish​

Keep a watchful eye on your fish during the treatment process. Medications can sometimes come with unwanted side effects. Fish take in those medications through their gills immediately, so they’re constantly exposed to it.

If you notice any side effects, stop treatment immediately to prevent any further damage.

The good news is that most fish will handle medications without any issues. Monitor your fish to ensure that the spread of the disease stops.

You’ll know this is happening when the frayed edges and discoloration stop getting worse. New growth will start to occur as well. In several weeks, your fish’s fins should grow back fully.

Tips For Prevention​

Fin rot can have a big impact on your fish’s health, appearance, and overall well-being. While it’s a pretty common disease, it also happens to be a preventable one.

A freshwater fish that recovered from fin rot

The best way to prevent fin rot issues in the future is to simply stay on top of tank maintenance. Adopt a regular cleaning schedule that includes getting rid of grime and adjusting water levels.

Every other week, consider replacing about 25 percent of the water. This can prevent ammonia and nitrate levels from going out of whack. Every week, use your trusty test kit to ensure that pH levels, hardness, and temperature are appropriate for your fish.

Another thing you can do is make changes to how your tank is set up. Avoid overcrowding and try to limit how much food you’re providing. If possible, remove some fish to give the community more room to swim around.

You could also try investing in a larger tank. More space is always welcome. It gives each fish more room, which reduces aggressive behavior.

As for feeding, only provide enough food that your fish can eat in a couple minutes (or as dictated by specific care guides). Remove any stray pieces after that. This will keep the tank in better condition for much longer.

If fin rot was caused by fin-nipping aggressors, remove them from the community tank. This move should be permanent, so take some time to set up an appropriate environment for the fish.

Author Note: As a good rule of thumb, you should never keep fish species that are known for nipping fins with slow-moving fish. Oftentimes, fish with large flowing fins don’t have the strength or speed to get away from these fish. Thus, they’re always going to be targeted.

To keep the peace and avoid issues like fin rot in the future, it’s better to just keep those fish separate.

Time To Get To Work!​

Now that you’re familiar with all aspects of fin rot, it’s time to start protecting your fish.

It’s actually not that hard to treat or prevent fin rot. All it takes is the right knowledge and a little commitment.

We hope you found this guide helpful. As always, feel free to reach out with any questions!
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