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What Is Fin Rot?
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What Is Fin Rot?

Fin rot is a common bacterial infection that affects many pond and aquarium fish. When caught early, it's one of the most preventable. Fin rot, sometimes called tail rot, is when bacteria eat away at a fish's fins and tail. Goldfish and koi are the two pond fish most susceptible to this type of bacterial infection. When left untreated, fin rot can take away an entire fin and infect other fish in the pond. Learn more about what fin rot is and how to prevent and treat it. 

What is Fin Rot?

When fish have compromised immune systems from stress, poor nutrition, or injuries, they are more vulnerable to infections. Fin rot is a bacterial infection. The bacteria eat away at the fish's fins, starting at the tips and making their way into the base. Once they infect the base, the bacteria can move into the fish's body and possibly cause death. 

What Causes Fin Rot?

The leading cause of fin rot is poor water quality. When fish live in water with contaminants and unbalanced water levels, it causes stress and weakens their immune system. A pond has poor water quality for many reasons, like overcrowding, overfeeding, or runoff. High levels of ammonia, nitrites, or too high or too low pH are harmful to fish. Sudden temperature changes can also contribute to fin rot, as it stresses fish. Bullying or aggressive fish can cause injuries that make other fish more at risk for fin rot, allowing the bacteria to enter wounds on fish. 

Fin Rot: What to Look For

In the first stages of fin rot, the edges of the fin whiten and lose their color. After the fin turns white and starts to die, it falls off. The edges of the fin become jagged or frayed. Red lines may lead to the fin base, leaving it looking "blood-streaked." As the bacteria works its way from the tip of the fin to the base, the area begins to redden and swell. Your fish may become less active and less hungry. 

How to Treat Fin Rot

The earlier you catch fin rot, the better. The bacterial infection is easily treatable, and the fin can grow back when found early on. The first step to treating fin rot is to improve water quality. Fin rot is most often a result of poor water quality. Perform a water test, paying special attention to ammonia, nitrites, and the pH level.  

Just One Infected Fish

If only one fish is infected, you can treat them in a tank separately with pond salts, and if the infection does not go away or is more severe antibiotics. Continue to work on improving the water quality in the pond and quarantine the sick fish. 

Place the fish in a separate tank with clear, balanced water and begin treatments. Add an airstone to the quarantine tank to provide dissolved oxygen, which helps the fish breathe, heal, and reduce stress. Beta fish prefer still water, compared to other fish, so place the air stone on a lower setting when treating a Beta. 

Gram-negative bacteria cause fin rot. The best fish antibiotics for treating these specific bacterial infections are chloramphenicol, oxytetracycline, and tetracycline. You may be able to find these antibiotics over the counter. If not, then through a veterinarian. Be sure to follow the instructions for the correct dosage. Many fish stores also sell medicated fish food designed to treat fin rot. 

Two or More Fish

If two or more fish in the pond show signs of fin rot, the infection has spread, and all the fish need treatment. You can start with improving water quality, then try pond salts, medicated fish food, and antibiotics if the infection is severe on multiple fish. 


Fish with compromised immune systems and poor water conditions are more susceptible to infection. Because their body is weak and they are stressed, it's not uncommon to have secondary fungal infections or parasites in addition to fin rot. You may have to treat two issues at once, like Cotton Wool and Dropsy, as secondary conditions to the fin rot infection. 

Preventing Fin Rot

Maintain Water Quality

One of the best ways to prevent fin rot is to perform routine water quality checks to maintain water quality. Pay special attention to ammonia, nitrite, and pH levels. Treating water quality issues is easier if you catch it early. Head to our water treatments page for more help with fixing water quality issues. 

Reduce Waste

Be sure that your fish aren't overcrowded or overfed. The excess waste hurts water quality and clarity. If you have a lot of leaves, grass clippings, and other debris making their way into your pond, you can use a pond net to help remove larger waste. 


A filtration system is essential for removing the tiny debris in the water that you can't scoop or use a pond net to remove. There are different types of filtration to address various issues and remove debris at the microscopic level. Learn more about pond filtration. 

Aerating water features like fountains or waterfalls oxygenate the water, promoting good bacteria growth. Adding plants to your pond also helps absorb nutrients and naturally filter the water. Read more about the best plants for your pond

Watch for Aggressive Fish

Injuries and stress can cause fin rot, often caused by bullying and aggressive fish in the pond. Some fish naturally are more aggressive, while others are more passive. Combining different personalities can be troublesome. Keep an eye out for bullying, and if one fish is being too aggressive and injuring or terrorizing your other fish, put them in a different pond or find them a new home. 

Use High-Quality Food

A key to preventing fin rot is keeping the fish healthy. High-quality fish food will help improve and maintain your fish's immune system, helping them to fight off infections naturally. Keep an eye on the expiration date. Stale and expired fish food may not have all of the vitamins and minerals your fish need. 

Spotting the signs of illness in a fish is often harder than in a dog, cat, or small animal. Many ponds have multiple fish, and there are many hiding places. You often may not notice signs of infection until it worsens or spreads and the symptoms become more obvious. Fin rot is a common bacterial infection that is easy to treat when caught early. The main cause of fin rot is poor water quality, injury, and a weakened immune system. You can treat a single fish in a quarantine tank, or if it spreads to other fish, treat the entire pond. When caught early, fish can fully recover and even grow some of their fins back.




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