Cotton mouth can be infectious, at the very least keep a sharp eye on all the other fish for signs.
There are medications at the pet store specifically for this problem, you could try those. Ask a salesperson to help you if you don't know which one to get. The medications can interfere with the biological cycle, test for ammonia a couple times a day after adding the medication. If ammonia stays at 0 for several days, you're out of the woods, but it is likely to go up, and require frequent water changes. (Don't forget to medicate the replacement water, based on the volume of the replacement water.)
Salt is often an effective cure for many types of diseases, I would recomend supplimeting the medication with salt. Use non-iodized (aquarium, kosher, or sea) salt. Add 1 teaspoon per gallon every 12 hours for a total of 3 treatments. (Note, this level can be damaging to plants, but is benificial to fish. This is a safe, cheap way to treat an unplanted tank.) Keep the salt treatment up for a couple weeks after the cure is effective, adding 3 teaspoons per gallon to your replacement water when you do water changes. After that time, the salt level will go slowly down with normal water changes. (If you only treated the
QT, besure to acclimate the sick fish slowly to his normal tank water and not just dump him straight back.)
Good luck!
Could you post your water parameters?