If one of your fish is showing ragged, whitish-edged fins, tissue that looks like it’s eroding, or fin edges turning grey and coming apart, you’re probably looking at fin rot. It’s the second-most-common bacterial condition in home aquariums after ich, and unlike ich, most cases are caused not by contagion but by poor water quality.

The good news: fin rot is treatable in almost every case if you catch it early. The bad news: most people reach for medications before doing the one thing that actually fixes 80% of cases — a big water change and a filter check.

This guide covers identification, first-response, UK-available treatments, and the underlying causes you have to fix or the fin rot comes back.

What fin rot actually is

Fin rot is a bacterial infection of fin tissue, typically caused by opportunistic gram-negative bacteria (Aeromonas, Pseudomonas) that live at low levels in every aquarium. Healthy fish with intact slime coats fight them off constantly. Fish under stress — poor water, injury, aggression, chronic ammonia exposure — can’t, and the bacteria colonise damaged fin edges.

That’s why fin rot is a symptom more than a discrete disease. The bacteria are usually already in your tank. The question is what’s letting them win.

Identification

Early fin rot:

• Fin edges look slightly ragged, as if trimmed with dull scissors
• White or grey rim along the fin edge
• Fins clamped tighter than usual
• Fish slightly less active than normal

Progressing fin rot:

• Fin tissue eroding backwards toward the body
• Reddening of the fin where the erosion is happening
• Fins turning opaque white in patches
• Small holes appearing in fin tissue

Advanced / body rot:

• Rot progressed to the body
• Ulcers or open sores near the base of the fin
• Fish lethargic, hiding, off food
• Secondary infections (fungus, columnaris) commonly appearing on top

Distinguish from:

• Fin nipping — clean bites, no rimming, no gradual erosion, no discoloration
• Physical damage (e.g. from decor) — usually one clean tear
• Columnaris — spreads faster, often with white cottony patches on the mouth/back
• Fungus — visible fluff, not tissue erosion

First response — before any medication

Nine out of ten fin rot cases in a UK community tank fix themselves with these steps and no medication at all.

Step 1. Test the water. Use an API Freshwater Master Test Kit — test ammonia, nitrite, nitrate.

If any of ammonia, nitrite are above 0, or nitrate is above 40 ppm, that’s your cause. Fix the water and mild fin rot cases reverse on their own within a week.

Step 2. Big water change. 50% water change with dechlorinated water at matching temperature. Use Seachem Prime at 5x dose — it detoxifies ammonia and nitrite while you fix the underlying problem.

Step 3. Check filter media. When was it last cleaned in tank water (not tap water)? Is intake clogged? Is flow noticeably slower? Rinse mechanical media (sponges) in a bucket of the water you just changed out. If you haven’t done this in 3+ months, do it now.

Step 4. Check stocking. How many fish, what species, what tank size? Overstocking is the underlying reason for chronic ammonia/nitrite spikes even after a proper cycle. If you’ve added new fish recently and haven’t upgraded the tank, that’s a strong hint.

Step 5. Check for aggression. Is one fish nipping another’s fins? Aggression causes physical damage that lets bacteria in. Common culprits: tiger barbs, some tetras (serpae, Buenos Aires), larger cichlids, male bettas with wrong tank mates.

If ammonia/nitrite are already 0, water changes are up-to-date, and no fish is being aggressive — then move to medication.

UK-available medications

Fin rot responds to broad-spectrum aquarium antibacterials. Not all are equal.

Best first-line option — Waterlife Myxazin. UK-formulated, broad-spectrum antibacterial. Safe with most community fish including catfish and loaches at full dose. Treats a 100-litre tank at about £8 per course. Follow the label — typically 5-day course with daily doses.

Alternative — NT Labs Anti-Fin Rot & Fungus. UK-made, targeted specifically at fin rot and fungus. Safe with scaleless fish. Popular with UK aquatic-shop staff for this reason.

Broad-spectrum alternative — eSHa 2000. Dutch, popular UK-side. Covers fin rot, fungus, mild bacterial infections. Colours the water briefly.

For severe or resistant cases — Kanamycin or Furan-2 (available from specialist UK aquatic sources, sometimes imported). Only escalate to these if the standard treatments fail after a full course.

Treatment protocol

Whichever product you use, the protocol is roughly the same:

1. Remove activated carbon from your filter before dosing. Carbon strips medications from the water within hours, wasting the dose.
2. Do a 25% water change first, so the water is clean going into treatment.
3. Dose according to label — do not increase for “faster” results. Overdosing damages beneficial bacteria in your filter and can crash the tank cycle.
4. Ensure oxygenation — add an air stone if you don’t already have one. Medicated water holds less dissolved oxygen; stressed fish need more.
5. Complete the full course — usually 5–7 days — even if fins look better after 3 days. Stopping early lets residual bacteria rebound and often produces a resistant strain.
6. Big water change at the end — 50% to strip medication residue.
7. Return carbon to the filter after the water change if you use it normally.

Community-tank concerns

Treating in the main tank vs a hospital tank. For fin rot, treat in the main tank — the bacteria are throughout the system anyway, and moving stressed fish stresses them more. The exception: if only one fish is affected and the others are large, expensive, or sensitive to medications, isolating in a hospital tank is worthwhile.

Sensitive tank inhabitants:

• Scaleless fish (catfish, loaches, kuhli) — use half-dose medications, or products specifically labelled “safe for scaleless fish”
• Shrimp and snails — most fin rot medications kill shrimp. Move shrimp to a temporary bucket-tank if you have to treat, or use eSHa EXIT which is copper-free and shrimp-safer.
• Plants — mostly unaffected by fin rot medications, though intense doses can slow growth briefly.

Salt. Aquarium salt at 1 tablespoon per 20 litres helps mild bacterial infections and boosts fish slime coat. Do not use with catfish, loaches, or truly scaleless species — they can’t tolerate it. In pure community tanks with only scaled fish, salt is a safe adjunct.

After treatment — preventing recurrence

Fin rot recurring within weeks means the underlying cause wasn’t fixed.

Water quality checklist:

• [ ] Weekly 25% water changes, dechlorinated
• [ ] Filter media rinsed in tank water monthly (not tap water)
• [ ] Stocking within tank capacity (rule of thumb: 1cm adult fish per litre; less for messy species like goldfish or plecs)
• [ ] Ammonia and nitrite always 0; nitrate under 40
• [ ] No dead fish or plant matter rotting behind decor

Aggression checklist:

• [ ] No fin-nipping species with long-finned fish
• [ ] Enough tank mates of shoaling species (schools of 6+)
• [ ] Adequate hiding places and territory
• [ ] No mismatched adult sizes (goldfish don’t belong with community fish)

Diet:

• [ ] Feeding proper amounts (what fish clear in 2 minutes, twice a day)
• [ ] Variety — not the same flake food only
• [ ] Sinking food for bottom feeders

Fix these once and fin rot rarely returns. Skip them and you’ll be treating repeatedly.

When it’s not fin rot

Neon tetra disease. Neon or cardinal tetras losing colour and swimming erratically, developing white patches under the skin. Not fin rot. Not treatable. Isolate and humanely euthanise; treat as covered in our fish disease overview.

Columnaris. Progresses much faster than fin rot — hours, not days. Often shows first as saddle-back lesions (white patches across the back) rather than fin edges. More serious; treatment window is narrow.

Fungus. Visible white fluff on fins or body. Often follows physical damage. Uses same medications as fin rot but with the addition of NT Labs Anti-Fungus if it’s specifically fungal.

Fin nipping damage (mistaken for fin rot). Clean bite edges, no rimming, no continued erosion after a few days. Fix by removing the aggressive fish, not by medicating.

Bottom line

Most fin rot in UK community tanks starts with water quality. Test first, water-change second, and only medicate if water is clean and the rot continues. Use Waterlife Myxazin or NT Labs Anti-Fin Rot & Fungus at full dose for 5–7 days, remove carbon during treatment, and finish the course even if fins look better.

Fin rot is largely a self-inflicted disease. Once you understand that, it’s mostly preventable.

Frequently asked questions

What causes fin rot in fish?

Fin rot is a bacterial infection that takes hold when fish are stressed by poor water quality, injury, or aggression. Fixing water quality reverses most mild cases without medication.

How do I treat fin rot?

First test water and do a 50% change. If it continues, treat with a UK antibacterial like Waterlife Myxazin or NT Labs Anti-Fin Rot & Fungus, removing activated carbon during treatment.

Will fin rot heal on its own?

Mild fin rot often heals once water quality is corrected. Advanced cases that have eroded toward the body need antibacterial medication to stop the infection spreading.

---

Related: common fish diseases overview, ich (white spot) treatment, how to cycle a fish tank. Setting up a hospital tank is one of the best-value things a UK aquarist can do — we cover the quarantine setup in our disease overview.

— Jaqline Smalling, FishForever