Fin Rot in Aquarium Fish

Causes, Treatment, and Prevention

Betta fish

Jane Burton / Getty Images

Fin rot is one of the most common diseases in aquarium fish, but it is also one of the most preventable. Technically, fin rot is caused by several different species of bacteria, but the root cause is usually environmental in nature and is often related to stress, which can weaken a fish's immune system. When fish are moved, kept in a tank with poor water conditions, subjected to overcrowding, or coupled with aggressive fish that chase them and nip at their fins, they are more susceptible to fin rot.

Fin rot can affect any type of aquarium fish, both freshwater and saltwater. While all species of fish are susceptible, fin rot is easiest to spot on fish with long, flowing fins, such as betta fish, angelfish, and many fancy goldfish. Due to the long length and delicacy of their fins, these fish are likeliest to suffer an injury that makes them susceptible to fin rot.

As the name suggests, the infection attacks the fins and tail of the fish, causing color changes, deterioration, and eventually, the loss of the fins and tail, which is life-threatening to your fish. Luckily, with treatment and tank maintenance, your fish can recover from this unsightly condition.

What Is Fin Rot?

There are many bacteria in a fish tank's water. Most of the time, these bacteria don't harm your fish, as the fishes' immune systems ward off infection. In fact, bacteria play an important role in the overall health of your aquarium ecosystem. However, when a fish is stressed, the immune system can be weakened, allowing bacteria to take hold and cause disease. The bacteria species likeliest to cause fin rot are Aeromonas, Pseudomonas, or Vibrio bacteria.

Symptoms of Fin Rot in Aquarium Fish

In the early stages of fin rot, the edges of the fins will discolor, appearing milky on the edges. Often this change is so subtle that it goes unnoticed until fraying of the fins or tail begins. As the infection spreads, small pieces of the fins die and begin to fall off, leaving a ragged edge.


  • Black, white or brown spots on the fins, tail, or the body
  • Ragged edges to the fins and tail
  • Fraying or torn fins and tail
  • Reddened, inflamed skin at the base of tail and fins
  • Complete loss of the infected fins and/or tail
  • Lethargy, lack of movement
  • Loss of appetite

Over time, the fins and tail become shorter and shorter as dead flesh continues to slough off the infected areas. The affected fins may become red and inflamed, including around the base of the fins, as more tissue is eaten away.

As your fish becomes more ill, it is likely to stop eating and become lethargic. This displays as less movement than usual or drifting near the bottom of the tank.

It is not unusual for sick fish to develop secondary bacterial infections. One such disease is columnaris, which is caused by another bacteria commonly found in fish tanks. If your fish is stricken by this bacteria as well as the fin rot, you'll notice cottony spots on the fish's fins and/or body along with the typical fin rot symptoms.

Symptoms of Fin Rot in Aquarium Fish

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Causes of Fin Rot

The direct cause of fin rot is infection with bacteria. However, the indirect cause of this condition is some type of stress that lowered the infected fish's immune system enough to allow the bacteria to take hold. There are many common causes of stress in aquarium fish.

  • Injury: The fins of your aquarium fish are delicate, and can be easily injured by a scrape along rough or sharp tank decor or plants, or by an aggressive nip from another tank inhabitant.
  • Overcrowding: The very rough rule of thumb for stocking a fish tank is 1 inch of fish per gallon of water. So for example, a 20-gallon tank could comfortably hold 20 1-inch fish, 10 2-inch fish, four 5-inch fish, and so on. Go much beyond that guideline, and your fish are likely to be stressed by the overcrowded conditions.
  • Aggressive Fish: Some species of fish are naturally more docile than others. If you combine aggressive and docile species of fish in the same tank, the docile fish are likely to become targets of aggression, causing them to be stressed.
  • Poor Diet: Feed your fish a quality diet of fish food formulated for their species to prevent malnutrition.
  • Poor Water Quality: The most common cause of stressed fish is poor water quality in the tank. Many chemicals need to be balanced in an aquarium to keep fish healthy. Too much ammonia, nitrite, phosphate, and other common chemicals can lead to stressed fish in poor health.

Diagnosing Fin Rot in Aquarium Fish

Fin rot can generally be diagnosed by the characteristic symptoms. If you see your fish developing progressively frayed fins and tail, particularly if the skin at the base of the fins is reddened, then you are dealing with fin rot. However, if you are uncertain, it is always best to consult with an aquatic veterinarian. They will test samples of your aquarium water for chemical imbalances that can cause stress and can generally diagnose your fish with fin rot based on its symptoms.


You must address the causes of the stress that led to the fin rot before your fish can heal. Start by testing the aquarium water. Check the pH and temperature of the water, and make sure they are appropriate for your fish species. Be sure there isn't excessive chlorine, ammonia, or nitrite in the water, and that the nitrate is under 40 ppm (mg/L).

A water change of up to 25 percent can be helpful in restoring healthy water balance, as can vacuuming the gravel substrate to remove food and waste debris. Take care to avoid overfeeding in the future.

Start putting dates on your fish food, as it loses the vitamin content fairly quickly after the food container is opened. Feeding your fish fresh, high-quality food in smaller quantities is far better than frequent, large feedings of stale foods.

Once the cause of stress is corrected, antibiotics will usually cure the disease itself. Treatment with a drug that is effective against gram-negative organisms is recommended. Although there are antibiotics and other treatments for sale in most aquarium shops, it's a good idea to consult with an aquatic veterinarian if you are uncertain as to the best antibiotic for your fish.

Always treat according to your veterinarian's specific directions, or follow the guidelines on the antibiotic packaging. Typically, you'll add a certain amount of antibiotic to the aquarium water based on the size of the tank. It's best to remove or turn off activated carbon filters during the treatment, as the carbon will otherwise absorb the medication. It is particularly important to continue treatment for the length of time recommended, as ending treatment too soon can result in a recurrence of the infection.

Prognosis for Aquarium Fish with Fin Rot

If you take steps to remedy poor water conditions in your fish tank, alleviate other common causes of stress, and dose your fish with the appropriate antibiotic, your fish has an excellent chance of recovering completely from fin rot. Generally, you'll see the fins begin to heal within a couple of weeks after starting treatment.

How to Prevent Fin Rot

Many of the measures to prevent fin rot are the same preliminary steps used to treat fish that have the disease. The best prevention against fin rot is good aquarium maintenance. Change the water regularly, vacuum the gravel, and monitor the water chemistry by having a regular testing schedule and documenting the results. This will allow you to quickly notice water chemistry changes that occur over time, giving you a chance to correct problems before they become serious.

When feeding, keep the quantity low. Only feed the fish as much food as they will consume in about five minutes, twice daily. Overfeeding is the most common mistake made by all fish owners, and contributes to the poor water quality that fosters bacteria. Purchase food in containers small enough that they can be used up in one to two months.

Do not overcrowd the tank, and watch for signs of fighting between fish that may damage fins. Take care when choosing tank mates for fish that have long flowing fins, as fin-nipping leaves fish more susceptible to fin rot. It is also important to keep water at the optimal temperature for your tank inhabitants.

If you suspect your pet is sick, call your vet immediately. For health-related questions, always consult your veterinarian, as they have examined your pet, know the pet's health history, and can make the best recommendations for your pet.
Article Sources
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