My betta Jazzie has always had one eye that's a little bigger than the other one. I assume it's some sort of birth defect, but he's really never had any serious problems and it gives him a bit of character,

He has developed popeye (or what I assume to be popeye) in that eye and nothing I've done seems to be helping it go away. I've used Maracyn in the prescribed dosage and for the recommended amount of time with no results. One site I found online suggested Epsom salts, so I added some to his water and that isn't helping. I even added a little Maracyn II that I had from a while ago to the water (I later realized it expired two months ago), but nothing. I treated him with Maracyn/Maracyn II on another occassion... is it possible he's just built up a resistance to it?
Has anyone else had a betta with popeye that's this stubborn? Any thoughts on what I can use to treat it now? He's eating and acting normally.
He's in a one gallon tank. I'm changing his water again tonight, so it's the perfect time to try a new tactic.
I appreciate any help or advice you could offer. Thanks!