What Is Fin Rot? (Answers and Solutions!)

Fin rot is a bacterial infection that eats away at a fish’s fins and tail. Fin rot is caused by a number of different bacterial strains, including AeromonasPseudomonas, and Vibrio bacteria. The major cause of fin rot is poor water conditions and overcrowding of fish.

I first started keeping tropical fish over 30 years ago. In that time, dozens of species have passed through my fish room and I have kept everything from the ubiquitous guppy breeding tank, through fancy goldfish to Oscars and African cichlids. No matter what species of fish I have kept, there has always been the odd occasion a fish became sick.

What Is Fin Rot?

Eating Daphnia
Eating Daphnia

Sadly, fin rot is fairly common in the fishkeeping hobby. There is no end to the number of threads on internet forums where people are asking for help because their fish’s fins are literally rotting away.

A few years ago a neighbor brought me round a guppy in a small cup which had fin rot so badly its tail and fins were non-existent and white fungus had begun to take over its body. Unfortunately, I had to euthanize that particular guppy.

Fin rot is the result of a bacterial infection that has got into a fish’s system. A fish’s major defense against any bacterial infection is its slim coat. If for some reason a fish’s slime coat has been compromised, bacteria will take advantage.

Fin rot is caused by a number of different bacterial strains, but AeromonasPseudomonas, and Vibrio bacteria are the most prevalent. Once these bacteria take hold, they can quickly overwhelm a fish and allow secondary infections to set in. Once a secondary infection starts it usually proves fatal for the fish.



What Are The Symptoms Of Fin Rot?

Fin rot is fairly distinctive. Once you have seen it, you can usually identify it again pretty quickly. Symptoms of fin rot include;

  • The ends of the fins or tail may turn a milky white or black color
  • Fins may begin to fray and look torn
  • The base of the fins and tail may become inflamed and red
  • Eventually, the entire tail or fins will rot away

Inexperienced fish keepers often miss initial signs of fin rot. The change in coloration can be very subtle at first. However, once the fins begin to rot away and they take on the frayed, torn look, fin rot becomes quite distinctive.



How To Treat Fin Rot?

If treated quickly enough, a fish’s fins can grow back. Over the years I have had a number of fish come into my care that were suffering from fin rot and providing we treat quickly and address the root cause, there is no reason a fish’s fins can’t grow back completely. I recently wrote an article titled Can A Guppy Fins Grow Back?.

In my experience, the best treatment for fin rot is Maracyn which is made by Fritz. Maracyn contains an antibiotic ingredient called Erythromycin. Erythromycin treats the fin rot but also any other bacterial infections, fungus, and diseases. Quickly Treating a fish suffering from fin rot seriously increases the chances the fins will grow back. You can check the current price of Maracyn on Amazon HERE.

If you are based in Europe, eSHa 2000 is a really effective treatment.

As well as treating the fin rot, it is important to establish the underlying reason the fin rot set in in the first place. The most common reason for fin rot is poor water quality. For the vast majority of fish, the following water parameters should be aimed for as an absolute minimum standard;

  • Ammonia – 0ppm
  • Nitrite – 0ppm
  • Nitrate – 40ppm or less

Desired water temperature and pH will vary massively depending on the species of fish you keep, but almost all fish in the hobby want the ammonia to be 0, the nitrite to be 0, and the nitrate to be 40ppm or less.

If you are unsure of your aquarium’s water parameters, a good quality test kit will help you. I personally use the Master Test Kit from API. I have found it to be reliable, accurate, and fairly easy to use.

If after testing you find your water parameters were not as good as they could have been, immediately carry out a large water change. Changing 50% of the aquarium water with freshwater will have an immediate effect on the health of your fish. A 50% water change should be carried out daily until the water tests are back in line with the expected parameters.

How To Prevent Fin Rot?

I would say prevention of fin rot is definitely better than having to cure it. The two biggest causes of fin rot are water quality and overcrowding.

As discussed above, all fish have a slightly different preference for water temperature and pH, but almost all fish want the ammonia and nitrite levels to be at 0ppm and their nitrate levels to be at 40ppm or less.

The only way to check these parameters is with an aquarium test kit. As mentioned, I favor the Master Test Kit from API. Every couple of weeks I can quickly test aquariums in my fish room to make sure the water parameters are where they need to be.

Frequent water changes are the key to keeping aquarium water clean. Even the best filters on the market can only take fish waste (ammonia) and convert it to nitrate. Filters DO NOT remove nitrate, so it is our job as fishkeepers to remove it by carrying out water changes.

Overcrowding is the other major cause of fin rot. When we have too many fish in an aquarium and the fish don’t have enough space, tempers can become frayed, and even docile fish can nip at one another.

This fin nipping can allow bacteria to get into a fish’s system. Overcrowding is also the quickest way to destroy water quality, which again will provide perfect conditions for bacteria to thrive and take advantage of your fish.

In Conclusion

Fin rot is a nasty disease that, if left untreated, can take over and kill one or more of the fish in your aquarium. Keeping the water clean is the key to preventing fin rot in the first place and quick treatment is essential if the bacteria does get a hold of your fish.


About the Author

I’ve been keeping, breeding, and showing tropical fish for nearly 30 years. Over that time I’ve done it all! I’ve had great success and I’ve made some really foolish mistakes (like the time I bought an Asain Walking Catfish). Read more…
Richard James
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