This is not a starters tip.
When we fish lakes and rivers, we often see carp that we have caught with deep sores on thier flanks and underbelly. These can be caused by a number of ways, Most often caused by spawning, attack by predators, anglers, scratching on sharp objects and some bacterial infections, the biggest cause is stress. If left untreated they most often can turn into an ulcer which after some time may well kill the fish, once the wound is deep enough the fish absorbs water and it literaly drowns.
These wounds are difficult to treat, and may not clear up completely, but with a little care may well recover.
I keep some largish carp, and in recent years i have had a couple that have had ulcers caused by spawning. These only started of as a cuts, but i never noticed them, by the time i did they were ulcerised. i treated them and cured the problem and luckily there were no visual scars after 12 months.
This is the simple method i used to do this.
After catching the fish i wrapped in a wet towel, Really you need two to do this as if it's a big carp you need someone to help hold it. I don't use any aneasthetic because i don't know how to.
If the wound is an old one, you may have to pull off any dead scales around the wound, these maybe harbouring harmfull bacteria. they generally come away quite easily, the dead scales have normally lost their colour and look paler and opaque than the rest.
Once this is done, gently dry the area around the wound and swab the wound with an antiseptic solution using a cotton bud, i use a medication called 'Malachite and Formalin', (you can buy this ready mixed from any aquatic store) once this is done, cover the wound with a generous covering of Vaseline, this to keep it waterproof, the Vaseline will wear off eventually. Thats it, it only takes a few minutes to do this, and it may well save the fishes life. Carefully put the carp back into the water and let it recover.
It worked for me, so it's worth trying