I'm glad that you have the fish separated. That was the next thing I was going to suggest if you didn't have a spare tank. At least that way no other fish can pick on the sick fish. And the others will if they get a chance. I once saw a ryukin take a chunk off of my sick calico fantail's fins.
I kinda freaked when I saw that.
That is a bit on the cold side for fancy goldfish. Most goldfish keepers recommend no lower than 60F for fancies because they are much more sensitive than the commons. Some of the more primitive fancy goldfish like plain old fantails might be able to survive lower than that, but I wouldn't want to risk it. Lower temps mean slower immune system. But are you sure the water is that cold? The east coast has been having 80F+ temps for the past month. If it's an outdoor pond, it's not likely to be that cold.
If the fish seemed clumsier at the store, then there could be some other problems with it you can't see. Buying a fish that seems "off" is never good on the wallet as most likely that fish is sick. I'm really interested to see this picture.