Can fish die from fungus?

I recently bought a Sarasa Comet Goldfish and a few minnows. So far all of the minnows have died, and the Sarasa (affectionately named Skittles) is well on his way to the same fate. Before the minnows died they developed a white furry substance on their skin, and their fins seemed to break off, now my Sarasa is starting to develope the same problems. I've researched this, and it seems to be a fungus. Could that be what killed my minnows? If so, what can I do to fix this problem? I've gotten attached to my Sarasa and I would hate to see him die as well.