What can I do for my sick Goldfish?

I had recently bought a couple goldfish. They are bigger than the others. They're about 7"+... Well, since they are fat and slow my sucker fish have taken a liking to their fins. I had taken out all the plecos and only left the all the goldies in the tank. Their fins are completely frayed (not rotten) and they have in the fins as well. They are loosing their color slowly and don't move much at all. I feel terrible. I know that they are EXTREMELY stressed out (why the fins are frayed) and I know the blood in the fins is from the plecos but I don't know what to do to make them eat and regain their health. Someone at Petco told me put Melafix... It's been 3 days using the product and their fins are looking a little better but they're still loosing color and wont eat. Help me!!!!?????