Goldfish losing scales?

Does your goldfish also have what appears to be open sores or maybe what looks almost like rugburn or bleeding? Some parasitic diseases can cause the scales to fall off. Fish lice, too, but mostly because they are rubbing against things to get them off. Is his behavior off too? Like swimming sideways or upside down? Most anti-parasitics cover a broad range of diseases, so you might be okay just to treat it with that for now. Wonder why it went away before...maybe the type of water you have been using got harder or softer and it's reacting to that. Fish are sensitive to a lot of things, so it may even be your tank-cleaning method, if that has changed in the slightest. I noticed too that you only have a 2-3 gallon tank for it. Fish will continue to grow into their environment, until they die, and goldfish have long lives. My cousin had a goldfish that lived 4 years and grew to the size of a guinea pig. Does your goldfish have a lot of swimming space? Maybe it is rubbing against something when it tries to swim and you just don't catch it. Good luck!

Velvet disease can cause your goldfish to lose scales. It is brought about by stress from poor quality water, changes in the water temperature, or being transported. Parasites under the scales can cause it to dislodge.

You can treat with copper sulfate for 10 days. At the LFS, get "Had-A-Snail" drops which should only cost around $5. "Had-A-Snail" contains copper sulfate (1 drop per gallon). If you can't find this, try Jungle Labs Binox which contains Nitrofurazone (very effective!)

Be sure to do frequent water changes.