Fin rot: Symptoms and treatment

by Owen James

Aquarium fish diseases are often rather ghoulish, and fin rot is no exception. It sounds like something from a zombie horror film for fish, and if you watch it progress in your aquarium you may feel like you’re witnessing the aquatic Day of the Dead.

Fin rot is treatable without a stake through the heart, however, by improving tank conditions and using one of several different treatments as discussed below.

Symptoms of fin rot

Fin rot is fairly easy to diagnose, though ideally you want to catch it when the disease has barely progressed as it will be much easier to treat.

  • The first signs of the disease are milky white areas appearing in the fish’s fins or tail, particularly around the edges.
  • The fins develop a rather ragged appearance as the disease begins to eat the tissue.
  • Eventually the disease eats all the clear fin membrane away, leaving just the fin rays.
  • If the fin rot has affected the fish’s tail (also called tail rot), it may work its way through to the body of the fish.
  • Secondary infections or diseases are common in advanced cases of fin rot, bringing new symptoms to the afflicted fish such as white cotton wool-like tufts or streaked patches of red.

If you catch fin rot early you can treat the root causes and let the fish take care of itself, which is always better than treating the whole aquarium.

Make sure you don’t misdiagnose a fish that just happens to have been bred to have frayed fins. Also, fin-nipping can cause a fin rot-like appearance in fish like guppies.

Causes of fin rot

Fin rot is a bacterial infection. It’s likely that several different bacteria cause similar diseases, with Flavobacterium columnare (previously known as Flexibacter) most often blamed. Aeromonas and Pseudomonas are also cited.

However it’s not really right to say the bacteria cause fin rot. Almost always fin rot only occurs in weakened fish, most often because of poor tank or pond conditions. Physical damage or an attack by parasites can also leave a fish susceptible to fin rot.

Fin rot often only manifests itself in one or two fish at once and it is not particularly contagious, adding more weight to the theory that a weakened immune system is required for the disease to take hold.

Treating fin rot

There are three ways to treat fin rot:

  • Environmental
  • Anti-bacterial treatment
  • Antibiotics

Fixing the environment

Fixing the environment is the most important element by far, since if the tank is in a bad way the fish will become reinfected with fin rot in the future, not to mention other inhabitants getting the same or other diseases. Fin rot is a wake-up call that you need to sort your tank out.

Do a couple of large 20-50% water changes with clean, aged water, and try to suck up as much debris and detritus as you can. Check and clean all your filters. Give the gravel a good clean, especially if you’re using an undergravel filter.

Look out for any tank bullies that are nipping or fighting with the other fish, and either remove them or the other fish. Similarly, if your tank is over-crowded you’ll need to remove some of its inhabitants to a new home.

Using an anti-bacterial treatment

Various anti-bacterial treatments and fin rot cures are effective against fin rot if caught early enough, such as Melafix (US link).

Treatments designed for aquariums or ponds should be safe if used according the instructions. If you can though, move the afflicted fish to a clean aquarium with a simple filter for treatment, rather than treating the whole tank. (Proper anti-bacterial fish medicines should not affect ‘good’ filter bacteria if applied in a proper dosage).

Treating with antibiotics

In the US it’s common to treat fish diseases with off-the-shelf antibiotics. Ask for a treatment that’s effective against gram negative organisms – there are several, which are sold under different names.

In the UK and other countries which have a more sensible attitude towards mankind’s best defense against diseases, antibiotics are typically only obtainable under guidance from a vet. As such, they’ll only be cost-effective with expensive fish, such as large koi or marine fish. Ask your vet.

Catch fin rot early enough and you shouldn’t need antibiotics anyway; better for you, the fish, and the world.

Preventing fin rot

Fin rot can usually be prevented by keeping your aquarium or pond in good condition, as discussed.

Set your tank up properly. Once running, check your tank or pond is sensibly stocked, and make sure you’re doing regular water changes (at least 25% a month, and ideally 10-20% a week). Consider whether you need to choose a new filter if your fish a messy eaters or you’re struggling to keep the tank clean.

Finally, diet has a crucial role to play in keeping fish fit and healthy. Make sure you’re feeding a wide variety of fresh foods, and that all the fish are getting enough to eat.

Read the Ultimate Secrets To Saltwater Fish And Invertebrates.

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{ 5 comments… read them below or add one }

randi 02.15.10 at 8:26 pm

Hi, when i came in today my oranda looks like it is in very late stages of fin rot. All of its fins and its tail are shredded and or have holes and he has both of the secondary symptoms that you describe in your last bullet point. I don’t understand because i checked on him the day before yesterday and he looked normal, no fin shredding at all. I don’t understand how the fish could have become so severe in such a short time. Is there any chance of survival? Will treating with something like Melafix be strong enough? Should I do anything to treat the secondary problems?

maggie 10.20.11 at 7:47 am

ive had two normal box standard gold with living in a bowl for a month without filter.have bought a tank with filter and bought one very small fan tail gold fish since then ive only seen once the small fan tail nipping got up this morning one of the normal gold fishes tails is a mess and the other one is starting to look a little nibbled do you think its the fan tail or could it be fin rot .i dont have the light on as it spookes the fish do you think i should leave the light may stop the little blighter eating his mates.i need a reponce as im away soon and dont want to come back to 1 fat fan tail any help would be great many thanks maggie

bri 11.27.11 at 12:12 am

hi i got a pineapple swordtail at a early stage of fin rot, i put the company tetra`s life guard in the tank with him, will he be ok? I have no other choice but to treat the whole tank and his mates right now to, there is also no gravel in there. Please help! =[

Will 09.21.12 at 1:48 am

randi:
I don’t know much about oranda’s, but if a betta is that far into fin rot, what you could do is this.
Get a paper towel damp, set your fish on it, and use a small paint brush to put peroxide on the tail. Keep it away from the face and gills so they don’t breath it in. Then you can put it in a little bowl, and let the peroxide rinse off. Put it back in it’s cage and put some aquarium salt in it to calm it’s stress. Just so you know this is in a very extreme case and only to be used if all the fins are gone. Tell me if it helped

Al Ulrich 11.27.12 at 3:24 am

I was searching the web for information about fin rot and found your article to be well written and quite informative, thank you for your contributions. I know you wrote this a long time ago–but it popped up in my google search. I think your description of the cause of fin rot being associated with (if not caused by) poor tank conditions is a great point. I think some people struggle with the idea that the environment we have created in our tanks is causing disease, but it is true. Thanks again for this good, clear post about fin rot.

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