The cold is the stresser more than likely. Use the salt, then large water change, and salt again.

Ich will be gone in a few days. Cheaper than over the counter treatments. Being goldfish though, I would add more than 4 ppt just do it slowly. You can google salt treatments for Ich and other versions of that phrase and see what others have done.
Faster you get after it, the better off you will be.