Hi Jake. Thanks for using the disease questionnaire!!
I'll address a couple of things here.
1st, the QT tank is a good size to deal with this discus. I'm glad you didn't place it in a community tank because you would risk making everything sick. Since this discus seems stressed and possibly sick, I would follow Al's QT procedure found here
http://forum.simplydiscus.com/showth...tine-Procedure
I would first give this fish a heat treatment. Turn the heat up to 93 for 10-14 days. This would also be a perfect time to treat with metro, preferably pure metro and not the over the counter stuff like General Cure. The heat alone may fix it up, but the metro is the med of choice when dealing with a discus with hex. I would also add 5 TBS salt per 10 gallons and do 50% water changes daily with aged, heated water. Give us a report if anything changes for the better or worse.
If this guy is still eating, you can also use a metro medicated flake food.
2nd, I recommend yo get yourself a larger tank for the day you have more discus. A 45g is much to small for 6 discus. I use 55g tanks for the most part because of limited space in my home, but a 75g is better for that many discus because they need at least 10g per adult fish.