Undecided Question

Sores on my goldfish?

I'm thinking that maybe it's fin rot but I'm not sure that fin rot can attack the actual body and gills? I noticed a piece of skin hanging from my black moors gills today and looked closer and there seems to be white/red sores all over her! On the beginning of her tail and her body where the body and tail meet. Also on her gill. I haven't noticed it on my other goldfish but in case it can spread I'd like to have it taken care of. It doesn't seem that the tail has been eaten away but it is a nasty looking sore… She's been hanging out at the top lately and not moving around much. She's also not been eating well and she's definitely been losing body mass. I admit my water conditions have been feisty lately and I cant seem to get them into an acceptable range as far as nitrates and such go. Should I administer an antibiotic or I'm not sure! Just a little advice?