Articles Fish disease

Treating fin rot

July 26, 2015
betta bowl bad

When the beautiful fins of your previously healthy looking fish suddenly seem to be melting away, it can be easy to panic. It may be fin rot! Luckily this common aquarium fish disease is often curable, especially if you take action right away.

Keep reading for more information on what causes fin rot and how you can prevent it as well as diagnosing and curing it if it’s already too late.

What causes fin rot?

Fin rot, like many fish diseases, is caused by a combination of factors. It’s a bacterial or fungal infection, meaning the “melting” is caused by bacteria or fungus eating away at the fins. Bacteria such as Pseudomonas that cause fin rot are not that dangerous for your fish by themselves and can actually be found in any aquarium. The real danger is secondary infection, such as when the fish rips its (tail) fin or is otherwise injured, for example by other fish. The bacteria and fungus can easily latch onto the open wound and cause it to become infected.

Another huge factor in fin rot is stress. A stressed fish is much more susceptible to any disease and especially fin rot! Stress can be caused by many things, but the main reasons for fin rot are bad water quality and an improperly cycled aquarium caused by overstocking or lack of water changes. Insufficient feeding, bad choice of tankmates, injuries, disease or stress from transport and all other things that cause stress and/or injuries to a fish increase the chances of infection. The oranda below has both dropsy and a case of fin rot on the pelvic fins.

Preventing fin rot

There are a few simple steps you can take to stop fin rot before it has a chance to start.

  • Keep your aquarium clean and ammonia and nitrite values at zero at all times. Make sure the tank is properly cycled before you introduce any fish.
  • Research the fish you’re interested in and their needs and compatibility to prevent aggression and overstocking. Although it’s tempting to buy as many fish as possible, a slightly understocked aquarium is much less stressful for both you (less maintenance) and the fish!
  • Once everything is up and running and the fish/invertebrates are introduced, do plenty of water changes according to your stocking level and the bioload of your fish, making sure to dechlorinate the water with a dechlorinator like Prime unless you live in a country that doesn’t use chlorine/chloramine in tap water.
  • Feed a varied diet of high quality food and prevent over- or underfeeding.
  • Avoid commercially bred very long finned fish such as veiltail goldfish or angelfish, halfmoon bettas and especially rosetail bettas. The large fins are easily damaged and bettas may actually bite off excessive finnage, leaving an open wound.

These steps will not prevent all cases of fin rot but if you always make sure to keep the stress levels of your fish as low as possible and try to prevent any damage/injury, the chances of infection are severely reduced.

Ragged fins can be a sign of fin rot. Photo by angelea

Ragged fins can be a sign of fin rot. Photo by angelea

Diagnosing fin rot

Fin rot is not difficult to diagnose. In the earliest stage, the fins may get red and inflamed looking streaks. This is not visible in all fish though! When the infection progresses, the typical “melting” of the fins begins. In severe cases and with long finned fish, this can happen very quickly, with entire pieces of the fins falling off. The fish may become lethargic and the fins will be left looking ragged and with white or dark-colored edges depending on the type of infection.

In very severe cases the fins can be eaten away completely. When this happens and the infection reaches the body it’s usually too late to save the fish, so it’s vital to act quickly when you see any of these symptoms.

Curing fin rot

In very early cases, increasing water changes and keeping the aquarium extra clean may be enough to stop fin rot. If the infection is already getting serious, it’s a better idea to start treatment right away. Fish that are in a community aquarium should be quarantined for this, as you don’t want to expose healthy fish to unnecessary medication. Keep the quarantine water very clean to prevent further deterioration of the fins. For more information about quarantining fish, check out this article!

If clean water isn’t working or the fin rot is already too severe, it’s unfortunately time to move on to stronger medication: antibiotics. These are not available over the counter everywhere, but in the US you should be able to find them. Maracyn 2 treats gram-negative bacteria that are usually the cause of fin rot and is a popular treatment for bacterial fish disease. With the right treatment you should see some fin regrowth in no time. The bettas pictured below are the same fish!

veiltail betta

Photo by Mason


Photo by Mason

Fin rot is one of the most easily prevented aquarium fish diseases. Although there’s always a chance you may end up with an infected fish after transport from the store, moving or an accident, keeping up with maintenance and doing research on proper aquarium care and tankmates should help you prevent almost all cases.

If you do spot fin rot on one of your fish, keep a very close eye on your water values and take the time to re-evaluate your water change schedule and the amount of fish you keep in the tank!

If you have any more questions about preventing and treating fin rot or if you want to share your experience, feel free to leave a comment below. Happy fishkeeping! 

Cover photo by The Blonde Aquarist

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  • Reply Satsuma February 25, 2016 at 3:43 pm

    I heard somewhere that Maracyn 2 is no longer being made! Does that mean I won’t be able to get it non-expired anymore, or was that just hearsay?

    • Reply Mari February 26, 2016 at 10:28 pm

      I’m sorry but I’m not sure! Maracyn 2 was never sold here but I included it because it’s commonly used in the US. Regulation is becoming stricter when it comes to antibiotics, though, so it’s unfortunately definitely a possibility.

      • Reply Satsuma February 27, 2016 at 3:17 am

        Thank you for replying! I was pretty worried about it so I did some research and concluded that to be on the safe side, I’d go with KanaPlex.

        • Reply Mari February 27, 2016 at 12:19 pm

          Good to hear you found a solution! I just try to keep medication to a minimum, but if I do dose it’s likely with Sera Baktopur or JBL Ekto Bac.

  • Reply Beauty is Pain. | Without Your Wings September 3, 2015 at 3:51 am

    […] as possible, but it might just be something I have to live with 🙁 I just hope he doesn’t get fin rot like Bruce came with; that was the beginning of the end for poor Batfish– the […]

  • Reply Daisy August 28, 2015 at 9:31 am

    I’ve been having massive issues with my half moon betta recently. He’s in a cycled 30L/ 8 gal tank (by himself) with a heater set to 26oC. I check the water weekly and the ammonia/ nitirites are 0 and nirates never go over 5, the PH is 7. I’ve been changing 30% of his water weekly and recently twice per week as I thought it would help the fin rot but it’s just gotten worse. What do i do? 🙁

    • Reply Mari August 28, 2015 at 10:39 am

      So sorry you’re having so many problems! You seem to be doing everything right so far but as mentioned in the article, if the clean water isn’t helping you’re going to have to switch to an antibiotic like Maracyn 2 before it gets even worse. Good luck, I really hope he recovers!

      • Reply Daisy August 28, 2015 at 12:38 pm

        Going to look for it tomorrow in the pet store and will order it if I can’t find it. Thanks so much for this post and your help 🙂

    • Reply Ilene August 29, 2015 at 4:18 am

      Sorry, my Betta had the same problem. We used everything, antibiotics for a month, and everything else everyone told us to do, he finally passed. We looked for Aquarisol, everyone said it would work. They don’t make it any longer. Good luck to you.?

  • Reply Alvin Yao July 27, 2015 at 11:25 am

    Don’t you mean, unless you live in a country that doesn’t use chlorine and/or chloramine in their tap water, not just chloramine?

    • Reply Mari July 27, 2015 at 11:30 am

      Oops, I’ll edit that. Neither is used here so it’s a bit confusing for me!

      • Reply Alvin Yao July 27, 2015 at 11:37 am

        ok then

  • Reply Ilene July 27, 2015 at 3:23 am

    Had a betta with fin rot we used everything but nothing worked he finally passed. Someone yikes us about Aquar-isol but they do not make it ny longer. Ht do you suggest if we decide to get bigger Betta.

    • Reply Mari July 27, 2015 at 11:34 am

      From what I can tell Aquarisol is meant to treat mild cases of ick and parasites, not fin rot so it wouldn’t have been effective anyway! If you get another betta and he unfortuntely catches fin rot as well, the methods listed in this article are most effective. A clean, cycled tank will usually prevent this disease though!

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