get some antibacteril meds like api's triple sulfa, tetracycline, erithromycin... now which one to chose depends on how the fish got the problem....if it from injury...step up wc's and keep water clean and maybe melafix will work. But in your case this looks like bacterial infection ....so I think the meds would be the best route....
I am not going to lie...trying to get rid of pop eye is a b!#CH!!!! and it may be a lost cause because of the infection being so advanced.
I hope for the best with your fish.