Fin rot in aquarium fish: causes and treatment

Symptom: Fins look ragged, frayed or shortened, with grey or white edges, red streaks or bleeding at the tips.

Likely causes, ranked

1. Poor water quality

very common

DiagnosticAmmonia or nitrite above zero, high nitrate, overdue water changes; several fish affected.

FixTest and correct the water first; ANGFA lists inappropriate aquarium conditions as raising susceptibility. See the ammonia page.

2. Fin damage from fighting or nipping

very common

DiagnosticOne or two fish affected, often long-finned fish with fin-nippers or rivals.

FixANGFA says aggression is a common trigger, with damaged fins opening the way to infection. Separate the fish or rehome the nipper; the stocking calculator flags known fin-nippers and long-finned fish.

3. Fungal infection, not bacterial

less common

DiagnosticANGFA: fungal rot shows creamy, hairy growths and spreads evenly; bacterial rot is whitish-grey and patchy.

FixTreat with a product suited to fungus, following the label, and fix the underlying cause.

Common myths

ClaimFin rot always needs antibiotics.

RealityMild cases can regrow soft fin tissue once the cause is fixed and the infection clears (ANGFA). The guide advises careful thought before any antibiotic.

Questions

What does fin rot look like?

ANGFA describes frayed fin edges and a reddened tail, then tips that turn greyish, erode and bleed; fish may stop eating and darken.

Will fins grow back after fin rot?

Often: ANGFA says mild cases regrow soft fin tissue once the infection clears.

Can salt cure fin rot?

ANGFA lists salt at 1–3 g/L for ongoing use in salt-tolerant fish such as rainbowfish, or short 20–30 minute baths at 10 g/L. Check your species' salt tolerance first.

Is fin rot contagious?

The bacteria are common in aquaria; fin rot takes hold where fins are damaged or fish are stressed, so fix those first (ANGFA).

How fast does fin rot kill?

ANGFA warns mortality can be high within days without prompt treatment, and rises sharply once bacteria reach the bloodstream.

Sources & references

1.
ANGFA Rainbowfish — Fin and tail rot, from Adrian R. Tappin, Rainbowfishes: their care and keeping in captivity (2011)
https://rainbowfish.angfa.org.au/disease_prevention_and_control/fin_and_tail_rot.html

Last reviewed 2026-11-05