Aquarium Fish Fin Rot

finrot on fish tail
Photo by Sue Waters / Attribution-ShareAlike 2.0 Generic (CC BY-SA 2.0)

Fin Rot Symptoms in Freshwater Fish

A fish with fin rot will have ragged, frayed fins. Both the fins and the tail may be affected. The edges of the fins are often discolored (sometimes lighter, sometimes darker).

If left untreated the fins will eventually erode away and the infection will spread to the fish's body.

Fin Rot Causes and Prevention

Fin rot is caused by a bacterial infection (e.g., Aeromonas, Pseudomonas). Sometimes fin rot begins after an injury to the fish's fins or tail. This could be due to fighting with other fish in the tank and having their fins torn or injured.

The bacteria that causes fin rot is normally present in the aquarium, but usually doesn't infect the fish unless they are injured or stressed in some way. Fin rot is an opportunistic infection. Once the fins are torn then a bacterial or even a fungal infection can easily invade the injured tail or fins.

Although any fish can get fin rot bettas and goldfish seem to be particularly susceptible.

More commonly fin rot is caused by poor water quality in your aquarium. In fact, most fish diseases are caused by poor water quality. Poor water quality stresses the fish making them more susceptible to ALL fish diseases.

To prevent fin rot from occurring in the first place don't overcrowd your aquarium or overfeed your fish. Also, don't keep fish that are known to nip fins, such as tiger barbs with fish that have long flowing fins (e.g., angelfish, bettas).

Even if you don't overcrowd your tank, have fin nippers, or overfeed your fish you still need to do regular partial water changes to prevent outbreaks of disease from occuring in your tank. This is very important. You'll often hear that you should do a partial water change once a week. However, I think that partial water changes need to be done more than once a week. At least twice a week and every other day is best.

Fin Rot Treatment

To treat fin rot the first thing you need to do is a partial water change. Change about 25-30% of the water, making sure to add a chlorine/chloramine remover to the new water.

If the fin rot isn't too severe, and the fish's fins are just a little ragged, doing partial water changes and using a product, such as API Melafix Antibacterial Fish Remedy may help. Melafix helps with mild bacterial infections.

You also need to add some freshwater aquarium salt to the tank. Don't use iodized salt - it is toxic to your fish. I usually use aquarium salt. It is cheap and lasts forever, however, you can use table salt as long as there is no iodine in it. The salt will help to inhibit both bacterial and fungal growth. Use about 1 tablespoon of aquarium salt for every 5 gallons of water.

Recommended Finrot Products

Often, just cleaning up the water and using Melafix and salt will cure your fish's finrot. I usually try to keep a bottle of Melafix on hand just in case.

Treatment for Severe Finrot

In severe cases of finrot, Melafix and salt may not be enough and you may need to use an antibiotic. For severe fin rot I used to use Mardel Products such as Maracyn or Maracyn 2.

Maracyn is for gram positive bacterial infections, whereas Maracyn 2 is for gram negative bacterial infections. However, most likely you won't know which kind of bacteria is causing the fin rot.

Unfortunately, it seems that these products are no longer available. For severe fin rot you'll probably need to use an antibiotic, such as tetracycline, which will kill both gram positive and gram negative bacteria. Only use it if you have to - it will discolor your water, plus the last thing we need is more drug resistant bacteria which often comes from antibiotic overuse.

For more information and for some excellent photos of fish fin and tail rot visit this page: