How to cure my golfish fast?

okay so i had 4 goldfish that got 2 weeks ago and noticed that one had his scales come up just like dry skin and then he had spots all over him and then he died so i took care of him and noticed that 2 days later my other goldfish had caught it so moved him to a different tank, but don't know what to do does this goldfish have the same thing as the dead one and are my other goldfish gong to get sick to and i need a cure quick i don't want them to die!!!! and what do cure for because i don't know the illness