Has he been showing any symptoms before today?
What are the
ammonia,
nitrite and nitrates in the tank?
What other fish are in the tank?
What's the temp. in the tank?
What heater, filter and aeration are in the tank?
What do you treat the tap water with and how often do you do partial water changes and gravel cleaning? When was the last time you did?
Do you have a pic of him to post? It would help be more certain that my initial thought is right/wrong and I hope it's wrong.
So I could be wrong, but unfortunately it sounds exactly like fish tuberculosis and you'll want to be careful to not get into contact with it as far as any open cuts, etc go. Limit skin contact and wear gloves. Everything will need disinfecting and/or destroying in the tank as it's contagious.
The options of what to do with the other fish depends on what fish you have. In the very early stages, fish tb can be fought with human tb meds with some success.
