That has some of the appearance of a cancer. It is also possible that it is
a non healing wound. Reasons for non healing wounds include wounds that are
kept open by parasites, high nitrate levels, or inappropriate attention by
other fish which might be grooming the wound. Some wounds just stay open until
you "annoy" and refresh the tissues (aka - debride) them.
Here's how that fish probably should be treated.
The fish should be caught and examined for other lesions, and look at the
gills. Photo-document the fish with a digital camera.
If you need to sedate the fish you would simply catch the fish and put it
into a large bin with aerated water and Oil of Cloves (four drops per gallon)
until it was "slow" enough to handle and photo-document.
The lesion should be scrubbed (without pressure). But it still gets
"scrubbed" using coarse GAUZE or a VERY-soft tooth brush - cotton balls or
paper towel are not abrasive enough and will slick over with fish slime - you
need something that would remove pretty much ALL of the loose scales, and
debris associated with the wound, refresh the edges and make it a bit
A soft toothbrush can be used, but whatever "scrubbing" textile or implement
you use should dipped in hydrogen peroxide FREQUENTLY while you're scrubbing -
An-n-n-nnnd - you can't put ANY real pressure on the wound.
I mean, you have to put SOME pressure on the toothbrush or gauze - but not
enough to erode the meat underneath that. You could deepen the wound
dangerously if you put pressure on the cleaning process, fish "meat" is pretty
soft in vivo and comes away rather easily.
Simultaneously (pretty much as soon as you're done debriding the wound) and
it's fresh but bloody looking - you should inject the fish with an antibiotic
like Baytril. Give that fish 0.4 - 0.6 cc IP
Obviously if you have another medicant like Azactam and you do NOT have Baytril, use the one you have according to dosing chart in the book.
For any help on the above topics (like cloves and injection) please search
out the articles on Koivet.com or see the book.
Replace the fish in the pond. Watch for improvement and consider re-injecting three days later with Baytril.
Do NOT - I repeat do NOT rescrub the wound until or unless we agree via
digital images that you should.
Complete failure to respond with the development of a tight white ring around the wound, and a slick pink center, would suggest that this is in fact a cancer, which are often induced by a combination of three things:
If it's a skin cancer, excision could be tried in that area because there is a LOT of space under that lesion which could permit margins for removal that the fish could easily live without.Footnote: I have tried this treatment and it actually works. Lawrence Belshaw, Fish Helpline.