Fin Rot: What Is It and How Can I Treat It?

Fish diseases can be a nightmare for beginner aquarists. If you go too long without water changes or let a fin nipping tank mate have their way, fin rot can be the result.

While the infection usually starts off small, if left untreated it can progress all the way to the body and even lead to death.

In this article I’ll explain what Fin Rot is and how to best treat this common fish disease.

Disclaimer: I do want to say that I am not a licensed veterinarian and this content is purely based on my personal experience.


What is Fin Rot?

Fin Rot is caused by one of two groups of microorganisms: bacteria or fungi. Pseudomonas, Aeromonas, or Vibrio are the bacterial organisms while any number of aquatic fungi can infect fish fins.

Knowing which organism is causing the infection is important because they don’t respond to the same medicines. Bacterial fin rot can be identified by inflamed blood vessels within the infected fins.

The rotting edge will be inflamed red or a pale, dead white, but usually has little material buildup. Frayed fin rays are sometimes left intact however the webbing is eaten away by the infection.

fin rot betta fish
Dizzy Respect / CC BY-SA

Fungal fin rot is quite different. You may see inflamed blood vessels in heavy infections but when the disease first progresses fins can look otherwise normal. However, cottony patches of clear or white growths are the hallmark of a fungal infection. Fungal Fin Rot tends to melt the entire fin, leaving nothing behind.

These growths can occur anywhere an open wound exists, including on the body, mouth, and even the eyes. However, fins are the most delicate tissues so we usually encounter infections here.

Over time, if the fish’s immune system can’t beat back the infection, the disease eats its way up the fin and can even spread to the body. Rotten or lost fins prevent fish from swimming normally, getting the food they need, and avoiding harassment.

Fin Rot is a slow death but easily fatal if allowed to progress this far. Fortunately, when caught early, Fin Rot is highly treatable and something most aquarists successfully save their fish from!

fin rot example
André Karwath aka Aka / CC BY-SA

Causes of Fin Rot

Fin Rot almost never simply appears. It usually takes the combination of an open wound and poor water conditions for it to arise.

However, fish kept in very dirty water can spontaneously develop Fin Rot (or other infections) without an obvious wound. High levels of ammonia, nitrite, and nitrate depress immune responses and make it possible for germs to take hold.

Bettas are especially prone to Fin Rot because their long, trailing fins provide plenty of surface area for infection to take hold. Also, they are usually kept in small bowls or tanks where overfeeding and infrequent water changes lead to prime conditions for bacteria and fungi.

In cleaner aquariums and community tanks, it usually takes an injury for Fin Rot to occur. Nippy fish, such as Tiger Barbs, may rip a small chunk of finnage from a Gourami, which may then become infected.

Rapid changes in water parameters can shock fish similar to how cold temperatures lead to an increase in the flu with humans. When performing water changes, are you ensuring that the replacement water is relatively close to the same temperature as the aquarium?

Making large adjustments of more than 3 to 5 degrees at a time can lead to Fin Rot, Ich, and other diseases.

Territorial squabbles are another common cause of Fin Rot. Cichlids, Bettas, Stingrays, and other fish prone to social jockeying, breeding matches, or territorial fights will often develop Fin Rot after their bouts.

The added stress of not being able to fully escape the aggressor in an aquarium also depresses their immune systems and increases their chances of infected wounds.


Product Commonly Used to Treat Fin Rot

In this section we will discuss a few specific products that can be used to treat fin rot:

Aquarium Salt

Sale
API AQUARIUM SALT Freshwater Aquarium Salt 65-Ounce Box
  • Contains one (1) API AQUARIUM SALT Freshwater Aquarium Salt 65-Ounce Box
  • Promotes fish health and disease recovery with increased electrolytes
  • Improves respiration for fish in freshwater aquariums

Aquarium Salt is a product nearly every aquarist should be using. It’s beneficial to nearly all fish and can be used in the treatment of nearly any disease. While it isn’t a cure-all, salt helps fish recover much more quickly!

The exact amount is a bit flexible and depends both on the species of fish as well as where you plan to treat them. Livebearers, rift lake Cichlids, and other hard water, salt tolerant fish should get a heavier dose when treating Fin Rot, while soft water fish like Discus and Bettas need less.

If you intend to treat the entire aquarium, start with a tablespoon per 3 gallons and increase up to one teaspoons per gallon over the course of 3 days. A tablespoon per 3 gallons is actually perfect as a general soothing remedy even when your fish aren’t sick.

Aquarium Salt boosts gill function and stimulates healthy slime coat production. Fish slime coats are a barrier to infection and a thin or damaged slime layer is an entry point for the germs that cause Fin Rot!

Once the infection begins to subside, use water changes to bring the salt levels back down to your preferred level. You should be doing water changes regardless the moment you see Fin Rot as germ-rich water is the primary cause.

A hospital tank can be left at one tablespoon per gallon indefinitely however you should acclimate your fish up to this level; do not simply move them from pure freshwater directly into this lightly salted setup.

While fairly effective on its own, Aquarium Salt will work far better when paired with one of the below proven remedies for Fin Rot!

Melafix

Sale
API MELAFIX Freshwater Fish Bacterial Infection Remedy 8-Ounce Bottle
  • Contains one (1) API MELAFIX Freshwater Fish Bacterial Infection Remedy 8-Ounce Bottle
  • Heals bacterial infections and repairs damaged fins, ulcers and open wounds
  • Contains natural, botanical tea tree extract to quickly and rapidly help fish

Melafix is one of my favorite products for treating not only bacterial Fin Rot but infected body wounds, inflamed gills, and other bacterial issues.

Melafix uses Tea Tree Oil, the same antibacterial agent found in Tea Tree Essential Oils used in aromatherapy and antibacterial/antifungal soaps. Melafix is formulated specifically for aquarium fish bacterial infections but is also effective for fungal Fin Rot.

The smell is instantly familiar once you open the bottle; a powerful antiseptic odor promising good results. Melafix will cause white foam to form at the surface of the water, which can be a bit alarming but is entirely harmless. The tea tree smell may pervade the entire room as well so hopefully you’re a fan of the aroma!

Follow dosing instructions carefully because Melafix is a fairly potent medication. Be particularly careful when treating scaleless fish, like Loaches, Stingrays, some Catfish, and fish fry or eggs.

While API says their product is perfectly safe for these delicate cases the fact remains that they tend to absorb more medication due to their lack of scales. A Melafix immersion bath might be worth trying first if you’re afraid to lose an especially delicate or expensive fish!

You also need to remove any activated charcoal in your filtration unit when using Melafix. As an organic oil, it will bind to aquarium charcoal and eventually be removed from circulation.

Pimafix

Sale
API PIMAFIX Antifungal Freshwater and Saltwater Fish Remedy 8-Ounce Bottle
  • Contains one (1) API PIMAFIX Antifungal Freshwater and Saltwater Fish Remedy 8-Ounce Bottle
  • Rapidly and safely treats fungal infections on body and fins of fish
  • Contains all-natural extract from West Indian Bay Trees to help with fungus and cottony growth

Pimafix is the fungus-specific product API made for fin, mouth, and body infections. Like Melafix, it is a plant-based medicine, based on the extract of the West Indian Bay Tree. Chefs and lovers of fine cuisine will recognize the distinctive aroma of Bay Leaf – often paired with seafood – and should find it less pungent and overwhelming than Melafix.

Again, remember that Pimafix is a powerful agent so consider carefully whether to treat an entire tank with scaleless or sensitive fish. An immersion bath or hospital tank may be the better option.

You also need to remove activated charcoal when using Pimafix to keep it in circulation as long as possible.

Furan-2

Sale
API FURAN-2 Fish Powder Medication 10-Count Box (70P)
  • Contains one (1) API FURAN-2 Fish Powder Medication 10-Count Box
  • Treats a wide variety of bacterial fish infections.
  • Easy-to-dose packets.

Furan-2 is a highly effective antibacterial remedy that treats bacterial Fin Rot as well as red, inflamed patches on the skin, eyes, and even interior infections. It is highly effective and is a product all aquarists should have on hand.

I strongly recommend using Furan-2 in a hospital aquarium because it is a broad spectrum antibiotic and can disrupt your cycling bacteria. While using it, continue to test ammonia, nitrite, and nitrate.

If you see a spike, do appropriate water changes while continuing the medication course and be ready to dial back feedings while your beneficial bacteria recover. Products like API Quick Start can help expedite the cycling process by directly providing new nitrifying bacteria!

Methylene Blue

Sale
Kordon #37344 Methylene Blue-General Disease Prevention Treatment for Aquarium, 4-Ounce, Blues & Purples
  • Kordon methylene blue 4oz
  • Methylene blue is effective against superficial fungal infections of fishes
  • The drug may be used as an alternative to malachite green for the control of fungus

Methylene Blue is an old school fish medication that’s gentle in comparison to Melafix or Pimafix and can be used to treat an entire aquarium with few concerns.

The only real issue is that it will turn the water blue for an extended period. It will also temporarily stain your skin navy blue (I ALWAYS manage to get some on me somehow) and can permanently stain clothing and other items.

Methylene Blue is specific to fungal infections so go with another agent if you have bacterial Fin Rot. This chemical also treats external parasitic infections such as Ich and Fish Lice and fungal infections on fish eggs.

However it ranges from safe to toxic to invertebrates and it’s not usually possible to know for sure how each species will respond beforehand. Anyone keeping snails, shrimp, or other creatures should avoid treating the entire tank with Methylene Blue and stick to immersions or a hospital aquarium.

Many live plants are sensitive to Methylene Blue and may be weakened by a tank-wide treatment.

Like the other agents, activated charcoal will bind and remove Methylene Blue so remove this from your filter before using.


Treating and Curing Fin Rot

There are three main ways of treating Fin Rot and I’m presenting them in order of least to most effectiveness.

I should mention that in all cases we want to raise temperatures a few degrees (3-5 degrees, but no higher than 84F) above the normal range to boost fish metabolisms and help them fight off Fin Rot.

Immersion Treatment of Infected Fish

For those with limited time and space, immersion treatments can offer fish a chance to recover from Fin Rot. I generally use immersion when I have a tank of sensitive fish or invertebrates that can’t tolerate the above medications but don’t have access to a separate hospital aquarium.

Immersion treatments should be stronger than normal, up to 50% stronger, but only for around 5 minutes. Water temperature should be identical; simply tank a small container of aquarium water and net the infected fish.

Place the fish within the bucket or jar, wait five minutes, and then return them to the aquarium. Immersion treatments can be done once a day for up to a week.

Of all the methods here immersions are the least effective as sustained contact with the medication helps it penetrate and kill germs better.

Treating the Entire Aquarium

Blanket treatment of aquariums is the most common way people treat Fin Rot and other diseases. So long as you don’t have other fish or invertebrates that will be harmed by Aquarium Salt or medications, this is fine to do.

Remember that activated charcoal will pull out all of the above medications except for Aquarium Salt. And that antibacterial chemicals (specifically Furan-2) can impact your aquarium’s nitrifying bacteria, which will cause cycling-related issues.

Moving the Infected Fish to a Hospital Tank

Hospital tanks are the best way to treat Fin Rot because the disease responds to treatment fairly slowly. It can take 1-2 weeks for inflamed or fuzzy fins to shed dead, infected material and heal enough for a fish to return to the main aquarium.

Hospital tanks allow you to dose heavily, alter water temperatures, and add liberal amounts of Aquarium Salt without affecting other fish, plants, invertebrates, or your filter media. And since hospital tanks are usually smaller, you need less medication and Aquarium Salt as well!

When your hospital tank isn’t needed for a sick fish, it can always be ready for breeding, raising fish fry, or even live foods like Daphnia or Fairy Shrimp!


Conclusion

Fin Rot is only lethal if we allow it to spread. Inspect your fish daily for signs of injury – Fin Rot is usually obvious. Pay special attention to fish that are prone to squabbles or have long, flowing fins.

So long as you leap into action with water changes and the right medication, your fish are sure to bounce back healthier than ever!

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