I'm sorry you lost your fish. If you would please answer the questions Byron asked it will help us along in understanding what went wrong here. I would also like to know if you saw anything on the fish that died? Sores? Fuzz? Redness? White spots? How many fish are remaining in the tank?
Byron is right about the fuzz. It's fungus. Fungus is dangerous because once it is internal it will eventually kill the organism. I suspect that is what killed the Goldfish
I'm sorry you received bad information from the pet store employees, but no Goldfish
can survive long in a 10 gal. The ones you see in the store are babies probably no older than a few weeks. The single-tailed ones grow around a foot long, and the double-tailed, bubble-bellied ones usually get about 10 inches. If you still have Goldfish
we can worry about tank size in a bit. Right now the main focus is on getting whatever is killing your fish cured.
PS. I also forgot to mention you can click on the shaded fish names to see information about them.