It's so hard to suggest any help with so limited information, Have you checked all your water checks, This should be the first thing you always check, second, have you added any salt? have you changed any water out? You said you have the fish out, have you tried to locate a vet or someone that is advanced at fish keeping that can look at it and get a a skin scraping to look at under a microscope? If the fish was wintered outside, this is when their immunity system is at it's lowest and some fish will show signs earlier than others. it's best to have your water checked and know what all your readings are, this will help in trying to figure out what is wrong.
A Common mistake made by many pond keepers, treating a fish for something you don't know what your treating for. Most fish illnesses are caused by stress due to water conditions, Fungus and bacterial infections are most likely to occur when the water is not at it's peak. parasites, which from the brief description it does not sound like a probable cause, attack the weakest fish first, All can be linked to water conditions. Best things you can do, Now that it is isolated, 30-50% water change using aged water, Water that has been sitting for more than 48 hours, dechlorinated water would be the second choice but not highly recommended as your now allowing chlorine/chlorimine plus what ever other chemicals that your water system has be exposed to the wound. Add salt, NON-IODIZED, Aquarium or rock salt. 1 cup to 100 gallons. Then FIND OUT WHAT IT IS. this is the trickiest part. positive identification is crucial before medicating, Is it contagious? Will it spread and how fast? Do you need to treat one fish or the whole pond. these answers can only be answered once you know for a fact what you are dealing with. BTW: All pics help!
