Fuzz on my fish

Question
Help me, please. I got a 100 gallon tank just before Christmas. I got it running (filter, chemicals, and heater) and then waited about 8 or 9 days before putting any fish into it. I have been adding fish pretty slowly; about 2 every 2 weeks or so. I have zebra danios, mollys, playts, 2 dwarf blue gorami, and algae eaters.
I have had a pretty good luck with them until about a week ago. Two danios, a molly, and a platy have died quite suddenly and today I came home to find a gorami with what looks like gray fuzz on its top fin and on its upper back on both sides. I have no idea what it could be or if it is going to kill all of my other fish. If you could give me some advice, I would really appreciate it. Thanks so much.

Answer
Dear Sherry,
  This sounds like Cotton Mouth Disease.  It is best to get this fish out of the tank ASAP.  The best treatment would be Methylene blue solution.  You can buy this from your local pet store.  Make sure to treat the fish in a separate tank from the others.  
  I would do a large water change before returning him to that tank.
You didn't mention a quarantine tank so I'm assuming you didn't set one up.  A lot of times a new fish introduction can also bring disease introduction.  Its imperative to quarantine new arrivals before adding them to your main tank.  You can never tell what might show up later on.  
 Hope I helped.