My fantail goldfish is very lethargic and will not eat.?

I just bought a few goldfish and they are all fine except the fantail. She is all white and very pretty, but she looks like she is full of eggs, but never laid any. At first, I thought she was bloating, but I'm guessing it's normal for then to be that shape. Anyway, she sits at the bottom of the tank and at first it was hard for her to swim, but she looks like she's getting stronger here and there. She won't eat that I have seen. The water PH level is fine and it hasn't effected any other fish in the tank. I have her in a separate tank now and I have given her a small amount of Epsom salt to maybe help with any infections or parasites. I don't think she has defecated, yet. I did get her and the others from Wal-Mart and I know they did not take care of these fish, so that might be part of the issue. Does anyone have any idea what is or could be wrong?