i lost some fish with this earlier this year. not sure exactly what it is.i think it maybe a bacteria infection? i used proform-c and it cured up pretty good. it will eat at the flesh all the way through. i had one that lost its tail and died, another that lost a good part of its mouth but he ok now. i feel for ya

jeff

Try koivet. They have a forum that might provide some help for you and your fish.

Here is a link that might be useful: koivet

That is truly an ugly wound. I have a comet that had that exact wound in that exact place. Another fish had two smaller wounds on one side. I was already treating the pond for parasites and I added an antibiotic. The wounds finally healed but there are white scars. Scales don't grow there. I remember being worried about tuberculosis and dismissing that for some reason. Maybe it was because of the second fish with wounds in different places. This all happened after a serious storm dumped a lot of rainwater from one of the hurricanes two years ago and lots of nasty things happened to the pond critters. Sandy

Thanks everyone. I've heard that too sleepless. It can be cured. If I use Proform C, it won't cure flukes, if that's what this is. If I use Supaverm for fluke, it will kill the only comet I have in the pond.
I guess I better give him an antibiotic shot see what happens.

I use Prazi for flukes in my pond, it doesn't hurt any of my fish. I have lots of comets. I also use Tri Cide Neo Dip. But you have to catch the fish to dip him. It is better than injections. It cured my fish that had a bacterial infection fungus and when the fungus dropped off it left his whole side with raw flesh. Looked like someone just pulled off all his skin. After the dip his skin and scales grew back and you can not tell anything was ever wrong with him. He is one of the most beautiful fish I have now. The following year he did get finrot and I used the dip again, and he recovered beautifully again. His name "Long Shot". Glenda

I have a 12 year old koi that had a open sore/wound similar in size to the one on your fish.
It was at the bottom of it's dorsal fin,on both sides of the fishes body.

I removed him from the pond and put him into a container with just enough water to cover him completely. About ten gallons of water.
I then dosed the water with Melafix, doubling the amount suggested in the instructions.
The second of the two teaspoonfuls of Melafix, I squirted directly onto the sore, using an eye dropper. This did not disturb the fish in the slightest.
I added two more teaspoonfuls of Melafix each day for three days.

After the third day we could see a definate difference in the wound. It did not look as red and sore as it had originally and seemed to be healing over.
Next day, I returned the fish to the pond and treated the pond for parasites.
Two weeks later, there was no trace of the open sore/wound, it healed totally and the fish is as good as new.
"Horton"