Well, I am fairly certain that I know what it is because my fish have it now! From what I have learned it is a bacterial infection that is HIGHLY contagious (it just looks like a fungus). The fish need to be treated with an anti-biotic/anti-bacterial solution. All fish in the tank must be treated too, since it is so infectious. I bought something called "Fungus Eliminator" which threats Columnaris, or Cotton or Fungus Mouth (the disease you probably have) as well as Ich and several other diseases such as fin rot. It was very cheap too, probably three dollars. You will need to do a 25% water change, then put in a teaspoon of the solution for every five gallons. It will turn the water bright yellow, so be warned! Then, after four days follow the same procedure. And don't forget to take out your filters and get new ones when the disease is cleared up. By the way, I found that this disease rarely spreads anywhere other than the mouth, and will eventually cause the fish's lip to rot off, or will release toxins which prevent it from eating and it will starve to death! Hope this helps!