Identifying fin rot
Fin rot is a general term for necrotic loss of fin tissue, resulting in
split or ragged fins. It is usually the edge of the fin that is attacked,
although occasionally a hole may appear in the middle of the fin. The
appearance of fin rot can vary between a distinct, semi-circular “bite”
shape and a “shredded” effect.
The edge of the lesion is usually opaque or whitish. In advanced cases
there may be some reddening or inflammation. The main threat from this
fish disease is, if left untreated fin rot can slowly eat away the entire
fin along with the fin rays and start to invade the fish’s body, leading
to peduncle disease if the caudal (tail) fin is involved, or saddleback
ulcer if the dorsal (top) fin is affected. Fin rot is a bacterial disease
involving opportunistic bacteria such as Aeromonas, Pseudomonas or Flexibacter
that abound in all aquatic environments. Secondary fungal infections are
Fin Rot (necrotic ulceration of fins)
click on pictures to enlarge them
A severe infection of the caudal fin of a koi.
There is extensive inflammation around the lesion extending into
the body of the fish. This fish recovered and surprisingly the
fin did partially re-grow, with the two halves knitting back
Typical fin rot affecting the dorsal fin of a
koi. On the leading edge of the lesion is an area of white
necrotic tissue. Surrounding this is a large area of inflamed,
infected tissue. The front fin ray has been exposed and destroyed.
The infection has reached the fish's body
A extreme example of fin rot of the caudal fin.
The white necrotic region of the fin edge is clearly visible. The
whole of the fin is very red and inflamed. The infection has
entered the body of the fish resulting in raised scales and
a large area of inflammation.
All of these fish recovered following fin
trimming, topical treatment and a course of antibiotics. The fish
in the top picture had his fin stitched. It held for a day or two,
which was long enough for the healing process to start. It was
either that or remove it!
Usually caused by stress
With very few exceptions, virtually all cases are precipitated by
stress, fear or poor environmental conditions. Indeed, fin rot is often
one of the first signs that a fish disease problem exists and all cases
should be investigated to determine the underlying cause. When I have fish
in for hospitalization, occasionally some sensitive fish will start to
develop fin rot as a consequence of their strange new surroundings and
being handled – even though they are being kept in optimum conditions.
It is usually self-resolving as they settle in, but does demonstrate just
how sensitive fish can be to stress and how fin erosion is often a sign
that all is not well.
At the expense of being repetitive, stress is the major cause of fin
rot. This could be due to a fish disease such as parasites, or
overcrowding, low oxygen levels, bullying, poor water quality etc. The
most important first step is to resolve any stressors. If caught early,
this may be sufficient.
In more advanced cases it may be necessary to trim the affected fin and
remove the necrotic tissue. This has to be done when the fish is sedated.
It is important to use sterile scissors and treat the clean edge with an
antiseptic such as povodine-iodine. This procedure does carry with it a
risk of secondary infection and it is important that the fish is returned
to optimum conditions and its progress closely monitored for the next week
If the fin rot has advanced the full length of the fin and is
threatening the body, this procedure would need to be accompanied by a
course of suitable antibiotics.
Does the tissue grow back? In minor cases where only the fin tissue is
affected it probably will, but in more advanced cases, particularly if the
fin rays (bones) are affected, the chances of re-growth are slim