Scale loss is almost always caused by injury, due to fighting with other fish, or from hitting sharp rocks in the tank. Goldfish do fight, so watch the one that isn't losing scales! Also keep an eye on the one that is losing scales, and make sure he isn't scratching up against the rocks. That could be a sign of a parasite like "Ich" which looks like little grains of salt on the body and fins.
Scale Loss- If your fish are losing scales for no apparent reason (such as breeding) then it is commonly found to be caused from Myxosporidiosis. The only treatment for this is to keep doing water changes and get rid of the parasites. Symptoms: Fish gasping, pale gills.
It takes about 8 weeks for scales to grow completely back, btw. Fungus is probably going to be the biggest danger, aside from the possibility of parasites.Salt would also help with preventing any kind of infection while the scales grow back... which takes a while.
From: Myron Kevus, DVM, http/www.aquarticles.com/articles/ponds/...Treatments.html ... Salt is also effective in treating injuries or infections to the slime coat of our pond fish. The slime coat is the clear, mucous, outer layer of skin on the fish. It is a vital protective outer coating that wards off infection. It acts as a liquid band-aid covering over minor scrapes and bruises so that they can heal more quickly.