This is going maybe a tad dark but what about something WWI themed if he's "mustard gas?"
Like Woodrow, Franz, Hemingway (was in the ambulence corps,) Patton, Kitchener, Norwest (famous sniper,) Joffre, Groener, etc. I like using last names as pet names sometimes. Or something military like General, Colonel, Sargent?
He's a very beautiful fish! He does remind me a bit of Wolverine, ha! That character's name was James Howlett/commonly known as Logan, so maybe one of those?