Guppy Cotton Mouth: Bacteria or Fungus?

[HannahJ]

So that pesky guppy that came in with ich also has cotton mouth and it passing it around the the other fish. Clearly this fish was a mistake, I will not be purchasing from that same store again.

Anyways, I need to dose for the cotton mouth before the whole tank gets wiped out. What do you think is best to use? I've seen conflicting reports on the internet as to whether cotton mouth in guppies is caused by fungus or bacteria, and I don't want to use the methylene blue "cure-all" treatment, as I would like to have inverts in this tank.

Tank info: it's an 8 gallon bowfront desk tank, with 4 guppies (one male, three females) and a platy. I brought in a new girl a few days ago and she's just caused all sorts of problems. No idea on the exact params, the test kit is at my boyfriend's place, but the tank is cycled and shouldn't be horrible. It's an established tank that's been running three months now, and I do ~40% water changes at least every other week. There is salt added (to deal with a touch of ich) and the temperature is ~88 F.

So, what should I use, do you think? Right now I'm also frantically watching my other tanks for signs of contamination, so this is not going to be a fun week.