I need some help here. I have 4 goldfish that i'm extremely proud of because I bought them as feeders and now they're about 6-8 inches long from nose to tail. The problem is, one of them has popeye, and i'm trying to treat him. The main tank is a 55 gallons, with 2 airstones and a filter (the water gets cloudy pretty quickly but I'm doing pretty frequent water changes now). I've quarantined the fish with popeye into a 20 gallon tank (yes, it has a filter too) and I'm trying an epsom salt treatment (about 2 and 1/2 tsp) to at least relieve the pressure. Any advice?

EDIT: update, i forgot to mention i have a powerhead in my main tank too which is powering an undergravel filter. i've turned that off for fear of nitrogen supersaturation. Also, i'm starting to think the quarantined fish with popeye also is beginning to get septicemia. can anyone help me out?

Update:

The fish seems to be stable, his popeye hasn't really gone down but it doesn't seem to have worsened either. I had him on a Maracyn-2 treatment for the 5 suggested days, along with Maracyn Plus and Stress Coat; and I havent added any Epsom salt to the tank in a while. I've been doing regular water changes, but now instead of adding the Maracyn-2 to the tank directly, I've been soaking bread and some flakes in a solution of Maracyn 2 and water, which the fish has been eating (I got this idea from a guy who did basically the same with his betta, it worked for him, but like he said theres no way to be sure how much is actually being ingested). I've been making sure that he eats the bread as fast as possible so that most of the Maracyn doesn't seep out. Also, there was bleeding in the popped-out eye, which has since stopped since the end of the Maracyn-2 tank treatment (hopefully that means any possible ulcerations have stopped themselves).