A gold fish alive in salt water! How? Why?

I have the little guy in with my fiddler (half land half salt water) and hes still alive! All the other types of feeders dies within a week but hes been in there for a month now! I also had another one in there that lived for over 3 months before it finally died but its only the gold fish that seem to surive...why is that? is it even logical? Since gold fish are fresh water.