Fin rot treatment: how to stop ragged, rotting fins and help them grow back

Fin rot is a bacterial infection that eats away at a fish's fins, usually starting at the edges, and it's almost always a sign that something has weakened the fish, most often poor water. The most important thing to do is test your water and do a large water change today; clean water alone cures many mild cases.

The bacteria most often involved, such as Aeromonas, Pseudomonas and Flavobacterium, live in every aquarium and often on the fish's own skin (ANGFA). A healthy fish fights them off. The Merck Veterinary Manual says control centres on removing the "predisposing factors": low oxygen, poor water, handling stress and injury. Caught early, fin rot is very treatable and the fins usually grow back.

How to identify it

Fin edges that look frayed, split or uneven, sometimes with a thin white, grey or dark edge.
A little redness or streaks of red in the fins, especially the tail.
Fins visibly shorter week by week, with ragged, milky or bloody edges.

Easily confused with:

Fin nipping: clean bites or chunks missing, with healthy-looking edges and no white or red rim. Watch the tank for a bully.
Torn fins from decor, nets or a filter intake. These heal in a week or two in clean water.
Columnaris: moves much faster, often with white or yellow patches on the mouth or back.
Chlorine burns: the vet manual lists ragged fins as a sign of chlorine exposure.
Fungus: fluffy white cotton growth on the damaged edge. It often follows fin rot.

When it's a problem

Early: frayed edges and a little redness, and the fish is otherwise acting fairly normally. Clean water is often enough.
Serious: rot reaching the fin base; exposed fin rays that look like bare spines; fins shrinking week by week.
Emergency: lethargy, clamped fins, loss of appetite, and ulcers on the body. At this point bacteria may be getting into the blood, and death rates climb sharply.
Normal, don't worry: a clear, thin, sometimes whitish edge of new growth on a healing fin. That's regrowth, not more rot.

What to do right now

Test the water: ammonia, nitrite, nitrate, pH and temperature. If ammonia or nitrite is above zero, that's very likely your cause (see ammonia spike).
Do a 30–50% water change with dechlorinated water matched to the tank's temperature. Vacuum the gravel.
Look for a fin nipper. Watch the tank for 10 minutes. Common culprits: tiger barbs, some tetras, and fish too big for their tankmates.
Remove sharp decor if long-finned fish are snagging on it.
Check the whole tank. If several fish have rotting fins, the water is the problem, not one unlucky fish.
Photograph the fins today, so you can tell in 3–4 days if it's improving.

What to do next

Step one: clean water (often enough on its own)

For mild fin rot, clean, stable water is the main treatment. Do water changes of 25–30% every 2–3 days for a week or two, keep ammonia and nitrite at zero, and keep nitrate low. Many mild cases stop spreading within days and start to regrow.

Salt

Salt is a common supportive treatment. The ANGFA guide says most tropical fish tolerate 1–3 g per litre continuously (1 g of fine salt is roughly a fifth of a level teaspoon; weigh it if you can), and that this level is safe for the filter bacteria. Use plain aquarium salt or non-iodized salt without additives. Dissolve it first, and raise it over a day or two.

Not everyone agrees on salt for catfish, plecos, loaches and other scaleless or soft-water fish. Some keepers use low levels, others avoid it. Many live plants and some snails don't tolerate it either. If you aren't sure, skip it and rely on clean water.

Antibacterial medicines

If the rot is still spreading after several days of clean water, or if it's moderate to severe, an antibacterial treatment may be needed. Shop products vary: some contain mild antiseptics, others antibiotics.

Antibiotics need a vet prescription in many countries, and the rules have been tightening. A vet can choose one that actually works on the bacteria involved. The vet manual recommends sensitivity testing when possible.
Bacteria from ornamental fish often resist common antibiotics (Declercq and others, 2013). Guessing and switching drugs every few days makes that worse.
Antibiotic food reaches bacteria inside the fish, but only if the fish is still eating.
Some antibiotics in the water can harm your filter bacteria. Test ammonia and nitrite every day or two during treatment.
Remove activated carbon during any medication. Follow the label or your vet's directions exactly, and finish the full course.

Herbal "natural" remedies are popular, but the evidence that they cure fin rot is weak. Don't rely on them for a fish that's getting worse.

Cautions

Shrimp and snails: some antibacterials and many "general cure" products harm them. Read the label.
Plecos, catfish and loaches: more sensitive to many medicines; use the label's guidance for sensitive fish.
Long-finned fish (bettas, fancy guppies, angelfish, long-fin plecos) get fin rot most often and need the cleanest water.

How long it takes

Expect the rot to stop advancing within a few days of good care. Full regrowth takes weeks to months. Keep the water extra clean the whole time, and finish any medicine course even if the fins look better. Fins lost down to the base may never grow back fully.

Products that help

Liquid test kit for ammonia, nitrite, nitrate and pH: finds the water problem behind most fin rot. Use it now and every day or two during treatment.
Water conditioner that removes chlorine: for the frequent water changes that are the main treatment.
Gravel vacuum: clears waste from the bottom during water changes.
Aquarium salt (or non-iodized salt with no additives): supportive treatment for salt-tolerant fish; skip it for sensitive species and planted tanks.
Antibacterial medication: for rot that keeps spreading after several days of clean water. Antibiotic versions often need a vet.
Medicated (antibiotic) fish food: reaches bacteria inside the fish, if the fish is still eating. Usually through a vet.

Keep on hand before trouble starts: a water test kit, water conditioner and a gravel vacuum. For most fin rot, that's the whole cure.

How to prevent it

Test the water weekly and do regular partial water changes.
Research tankmates before you buy. Keep long-finned fish away from known nippers.
Don't overcrowd, and don't overfeed.
Quarantine new fish, so you're not adding a sick or stressed fish to a healthy tank.

Why it happened

Poor water quality: ammonia, nitrite, high nitrate, or infrequent water changes. This is the most common cause.
Fin damage from nipping tankmates, rough decor, or handling.
Stress: overcrowding, temperature swings, another illness.
Overfeeding, which fouls the water (see overfeeding).

When to get help

Get help if the rot reaches the body, if you see ulcers, red streaks in the body or bulging eyes, if the fish stops eating, or if it keeps spreading after a week of clean water. A vet who sees fish can take a swab, choose the right antibiotic, and prescribe it legally. A good fish store can help you work out why the water went bad. Honest limit: once infection is in the bloodstream, even good treatment may not save the fish.

In the game

In Multiple Tank Syndrome, the Fin rot card is "bacteria eat ragged fins": long-finned fish and any fish with nipped fins in the hit tank die. Antibiotic food or the Medicine basket saves the tank. In real life, clean water saves more fins than any medicine.