I mean no offense by this, but just a tip from my experience.
I've had Koi all my life; they are my pride and soul. And whenever I have to treat any of them for ulcers or fin rot or anything, I have always been successful using latex or rubber gloves dipped in tank water instead of a towel. Only because a towel is very likely to damage their slime coat which is detrimental in a time of stress such as having something stuck in their mouth :P The latex or rubber gloves (Not dry though! Must be wet with tank water - in my case pond water) help to reduce any removal of the fishes slime coat.
The reason I say gloves and not your bare hands is because our natural oils can eat away at their slime coat also. And its cleaner for you that way too; also very easy to hold the fish down
And placing the fish down on plastic wrap (saran wrap) which is wet with aquarium or pond water helps too (instead of a dry surface such as a table or plate or whatever you choose to use).
Just some of my personal experience for others to use down the road