Why Is My Goldfish Furry What Is Wrong With My Goldfish?

What is wrong with my goldfish? - why is my goldfish furry

I have a tank of goldfish in the world, I bought a new one the other day and now all the fish have died and a bar which is covered in a sort of white / yellow sweet things about him. I pulled the tank and into a small store and tried all kinds of fish processing. Please heal, can not help this thing and how to get rid of the tank