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Aquarium Fin Rot

Fin Rot

Updated on February 03, 2023
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Reviewed by Dawn Burch
Director of Animal Care, Education & Compliance (ACE) at Petco

Jumping into the fish-keeping hobby can be a great experience. While some fish are considered low-maintenance pets, certain health issues can occur when an aquarium is neglected. Since your finned friends from the live fish shop can be impacted by serious bacterial and fungal infections, it’s important to be able to identify and address these diseases early to help improve your fish’s chance of recovery.

Aquarium fin rot can affect both freshwater and marine fish for a few reasons, however, most of them are environmental in nature. Maintaining proper water quality and keeping fish in low-stress situations are integral to your pets’ health and wellness. Let’s talk about why fin rot happens and how to help prevent it.

What causes fin rot?

Fin rot is typically caused by bacterial infections, which can occur when a fish’s immune system is compromised. This can happen when fish are stressed, from poor water quality, high levels of ammonia or nitrite or a lack of oxygen. 

Fin rot basics

Fin rot is one of the most common and preventable bacterial diseases in the aquarium hobby. Once a fish’s immune system is compromised, they become susceptible to contracting bacteria. Tropical fish fin rot is caused by several types of gram-negative bacteria that can result in a secondary fungal infection. Fin rot can be caused by aeromonas, pseudomonas and vibrio bacteria.

What does fin rot look like?

betta fish with fin rot

In the early stages of fin rot, a slight milky discoloration and/or deterioration of the edges of fins and/or the tail can be seen. Often the beginning stages of fin rot go unnoticed and it’s not until fins begin to further fray and deteriorate in the later stages that pet parents really take notice. White, cottony fungal growths may also develop if left untreated. If not caught until the later, more advanced stages, lengthy treatment and recovery periods may be required.

Common fin rot treatments

Fin rot treatment varies depending on the severity of the infection and whether you’re caring for freshwater or marine fish. The first thing you’ll typically need to do is assess your aquarium’s environmental health. Reviewing your water’s parameters using a water testing kit along with visually inspecting the filters and substrate are great starting points. Some basic fish tank maintenance may help eliminate the sources of the bacteria.

Changing about 25 percent of the water in the habitat might also be helpful. A total water change may only add to your pets’ stress and is not recommended, but poor water quality is likely contributing to bacterial growth. A partial water change is a good way to help minimize impact to the nitrogen cycle (and the onset of new tank syndrome) and not put additional stress on your fish.

With less severe cases of fin rot in fresh water, treating the aquarium with a combination of a partial water change and freshwater aquarium salt can be very effective with limited side effects. Always use a refractometer to measure the specific gravity if adding freshwater salt to your aquarium or conducting salt baths/dips.

With less severe cases of fin rot in a marine aquarium, freshwater baths or dips can be very effective while preventing antibiotic resistance.

In severe cases, you may need to use a broad-spectrum antibiotic to kill the bacteria in the aquarium and resolve existing infections. Treatments used to help eliminate aquarium fin rot include erythromycin, MelaFix and tri-sulfa tablets. Should the fish exhibit signs of a secondary fungal infection, additional treatments such as Maracyn Oxy or PimaFix may be required. Most solutions are available over the counter, but many antibiotics require a prescription from your veterinarian.

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What does fin rot look like?

Fin rot often happens in stages, getting more severe as the infection progresses. It’s important to note that while fin rot often appears on the fins or tail, as the name suggests, it can affect other parts of your fish as well. Red spots anywhere on a betta fish’s body, for example, often indicate fin rot.

Common early symptoms of fin rot in aquarium fish:

  • The fin or tail will begin to have frayed edges
  • Discoloration will occur in the fin or other parts of the fish’s skin, including spots or black and brown fin edges
  • The base of the fin may become inflamed
  • The fin or tail will eventually rot off altogether, a sign that the infection has reached a life-threatening level of severity

How do fish get fin rot?

Fin rot in betta fish and other freshwater and marine fish is caused by a bacterial infection. Once a fish’s immune system is compromised, it makes them susceptible to contracting bacteria.

Such stress factors as poor water quality, aggression, damaged fins, poor nutrition and improper temperatures are primary causes. Many fish who come down with this infection have recently moved, were living in an overcrowded aquarium, were being overfed or had been eating expired flakes.

Poor water quality allows bacteria to thrive and multiply. If you suspect that any of your fish have contracted fin rot, visually inspect the aquarium for excessive waste or fish food amongst the substrate. Immediately performing a partial water change while siphoning the substrate will often correct root causes. Also, be watchful of more aggressive inhabitants in your aquarium who may be nipping at fish, as this may cause more susceptibility to fin rot.

Is fin rot contagious to other fish?

Yes. If one infected fish comes into close contact with another fish, the bacterial infection may transfer to a new host. Therefore, the disease is commonly and collectively referred to as aquarium fin rot. If you have one sick fish in a habitat containing many, quarantining them until the infection clears up is recommended.

If all your fish become infected, there is likely a large-scale environmental issue at hand. Fin rot antibiotics can help clear up bacteria in the entire aquarium.

How long does it take to recover from fin rot?

Once an antibacterial medication is added to the habitat and other remedies are applied, you should notice your fish healing within a few weeks. While the bacteria may be eliminated faster than that, it takes time for your fish’s fins and skin to get better. No change after two weeks could mean you need more aggressive treatment. Consult your veterinarian if initial treatments don’t lead to recovery.

Can I cure fin rot with water changes?

A complete water change is rarely ever recommended. Instead, thorough cleaning of the aquarium and systematic removal of waste and debris through partial water changes should be undertaken while other remedies such as salt baths/dips (for freshwater fish) or freshwater baths/dips (for marine fish) and/or antibiotics are administered.

On the other hand, when the fin rot is caught early, a water change with the addition of aquarium salt (for freshwater fish) may be enough to stave off the need for medication. When in doubt, talk to an aquatic specialist or your veterinarian about whether a full or partial water change would help or hurt your pets’ situation.

How can I tell the difference between fin rot and nipping?

Sometimes one fish will be a dominant aggressor toward the other fish in your habitat and nip their fins. Other fish may swim by sharp aquarium décor and tear their fins. How do you tell the difference between fin rot and another injury? Here are some red flags that you’re dealing with a bacterial infection and not just fin trauma.

  • Nipping from another fish can tear off a piece of the fin, but it won’t cause discoloration. Only fin rot will turn the edges of your fish’s fins white, brown, black or another color.
  • Nipping and fin trauma will heal quickly. Meanwhile, fin rot can cause your fish to be listless and start rubbing against décor, worsening the infection.
  • Nipping does not spread to a fish’s skin. Aquarium fin rot can eventually spread to the rest of your fish and cause spotting or noticeable deterioration, but a nipping injury will not typically cause that.

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