Fin rot: Identification & Treatment
Fins appear shredded, milky at the edges or shortening over time — bacterial infection often follows injury or stress.
Fin rot
Fins appear shredded, milky at the edges or shortening over time — bacterial infection often follows injury or stress. Compare your fish with the reference photo above and cross-check your logged parameters in Aquarium Tracker before medicating.
What causes fin rot?
- Poor water quality allowing secondary bacterial infection
- Fin nipping from incompatible tank mates
- Physical injury from sharp décor or handling
- Chronic stress from overcrowding
What to check before medicating
| What to measure | Typical target | What an off reading points to |
|---|---|---|
| Ammonia (NH₃/NH₄⁺) | 0 ppm | Anything above zero means the biological filter cannot keep up. Toxic to fish long before it is high enough to feed algae. |
| Nitrite (NO₂⁻) | 0 ppm | A reading above zero points to an immature or disrupted filter — common after a media change, medication, or a large clean. |
| Nitrate (NO₃⁻) | 5–20 ppm planted · under 10 ppm reef | Climbing nitrate signals more waste going in than water changes and plants export. Near zero in a planted tank starves plants and favours algae. |
| Temperature | within your species range, steady | Warmer water holds less oxygen and speeds both metabolism and pathogen growth. Sudden swings stress fish more than a steady figure at either end. |
| Stocking level and mix | suits volume, footprint and temperament | Crowding raises waste and stress together, which is why disease often follows a new addition rather than a parameter change. |
| Recent changes | note anything from the last 2–3 weeks | New livestock, plants, media, lighting or dosing are the usual trigger. The cause often precedes the symptom by a fortnight. |
Take these readings first. A large share of what looks like disease is a water quality problem, and medicating a tank whose ammonia is climbing treats the symptom while the cause keeps working.
Treatment steps
- Improve water quality with frequent partial water changes
- Remove or isolate aggressive fish if nipping is observed
- Aquarium salt may help some freshwater species — verify species tolerance first
- Antibacterial treatments are available but water quality must be addressed simultaneously
- Severe cases need isolation and professional advice
Prevention
- Keep ammonia and nitrite at zero — most bacterial and fungal problems follow a water quality lapse rather than arriving from outside
- Remove sharp décor and address bullying; these conditions take hold on damaged tissue
- Avoid overstocking; crowding raises waste and stress at the same time
- Feed only what is eaten in a few minutes, and remove the rest
Frequently asked questions
Is fin rot contagious to my other fish?
Not usually passed fish to fish. It follows damage or poor water quality, so tank mates are at risk from the same conditions rather than from the affected fish.
How quickly do I need to act?
Act within a day or two. Most conditions are far easier to reverse early, and a fish that has stopped eating has usually been unwell for longer than it has looked unwell.
Should I treat in the display tank or a hospital tank?
A separate hospital tank is preferable where the fish is well enough to move: doses are easier to control, and many medications harm invertebrates, plants or the biological filter. Treat in the display only when the whole tank is affected or moving the fish would add more stress than the treatment removes.
Related guides
- Open sores and ulcers — easily mistaken for this
- White fuzzy patches — easily mistaken for this
- Every symptom, side by side — if you are not certain what you are looking at
- Quarantine protocol — what most outbreaks trace back to
- Choosing a test kit — for the readings above
