If it is columnaris, I had success with maracyn and maracyn 2 together. I treated the tank for 10 days, doing a 50% water change after 5. I also lowered the temp to 75-76 because the bacteria thrive at higher temps. I would also suggest treating the entire tank because columnaris (if it is columnaris) is very contagious and moves fast. Do some more research, look at pics and compare them to you fish. Columnaris not only shows up on the mouth, it also shows up other places. My fish had white saddle back type lesions on their backs.
Watch your bio-filter. It may crash.
I also had an Apple snail in the tank and it was fine.
There's a good article here......
http://www.aquatic-hobbyist.com/profiles/disease/freshwater/columnaris.html
None of my fish have lesions, no where. just the one fish and it looked like a cotton ball by his mouth. I took it off. I just did another water change and that tank is spotless. Do you have pics of this stuff? ( cotton mouth) I am just not sure thats what he has, the other fish do not show any sign of cotton, or lesions. but there tails are very short so tail rot comes to mind.