How to Treat Fin Rot in Aquarium Fish (Step-by-Step Guide)

Fin rot is a bacterial infection that eats away at a fish’s fins, leaving them ragged, frayed, or discolored at the edges. It is one of the most common diseases in home aquariums, and it is very treatable: improve the water quality that allowed the bacteria to take hold, then treat the infection itself — typically with a broad-spectrum aquarium antibiotic such as amoxicillin. Caught early, most fish regrow their fins completely.

What does fin rot look like?

• Fin edges that look ragged, torn, or “melted” rather than smooth
• White, black, or brown discoloration along the fin margins
• Fins that get progressively shorter over days or weeks
• In advanced cases, redness or inflammation at the fin base — a sign the infection is reaching the body

Fin rot is usually caused by common aquarium bacteria that attack fish whose defenses are lowered by stress — poor water quality, overcrowding, aggressive tank mates nipping fins, or sudden temperature swings.

How do you treat fin rot?

1. Test and fix the water first. Check ammonia, nitrite, and nitrate. Do a 25–50% water change, vacuum the substrate, and remove any decaying plant matter or uneaten food. Many mild cases start improving from this step alone.
2. Move the fish to a hospital tank if you can. Treating in a separate tank protects your main tank’s biological filter and lets you dose accurately. (See our hospital tank guide.)
3. Remove activated carbon from the filter — carbon absorbs medication and stops it working.
4. Treat with a broad-spectrum aquarium antibiotic. Fish keepers commonly use amoxicillin (such as Fish Mox 250mg) for fin and tail rot — both strengths and dosing basics are covered on our Fish Mox page. Typical label directions dose one capsule per 10 gallons every 24 hours with partial water changes between doses — always follow your bottle’s label. For stubborn cases, keepers sometimes step up to combination sulfa treatments like Fish Sulfa.
5. Complete the full course — usually 5–10 days per the label. Stopping early lets the infection rebound.
6. Finish with a water change and fresh carbon to clear residual medication.

How long does fin rot take to heal?

With clean water and proper treatment, you should see the fin edges stop receding within about a week, and regrowth — often clear or pale at first — over the following two to four weeks. Severe cases that reached the fin base take longer and may leave some permanent shortening. If a fish shows no improvement after a full treatment course, reassess the water, confirm the diagnosis (fungus and nipping damage look similar), and consult a fish-experienced veterinarian.

How do you prevent fin rot?

• Keep ammonia and nitrite at 0 ppm with regular water changes
• Avoid overstocking and fin-nipping tank mate combinations
• Quarantine new fish for 2–4 weeks before adding them to the display tank
• Feed quality food in amounts the fish finish within a couple of minutes