Okay the treatment now

sorry had to actually read here for a second and understand some of the meds and what is involved to get rid of this one. I will tell you right now I have never had this issue in my life, just know about it and how to identify it and the treatments. With discus being expensive as they are you have to be 100% on what you are going to do. I am going to take it little by little on what you can do. I have all the information here just need to type the important stuff for you out here. And really look at getting yourself a book about them to help you out in rough situations and this would be a rough situation
First Thing
A prolonged bath in potassium permanganate solution, daily in a 1:5000 dilution by weight remember by weight. This is very important. Its about 75mg/gallon 3.8l remember everyone's gallons is different so 75mg meds/ 3.8Litres of tank water. Remember to have the bowl clean very clean this is an oxidizer med and the bowl needs to be very clean I can't stress this hard enough. You are going to do this daily although what a poor book this is, it don't tell you how long to do the treatment for. Read the bottle it says that they are given out in different dosages for differnt animals. Follow what it says for time, lol I just read that now. Want to make sure I give all info to you that is really important.
Next thing
Sodium Chloride(Salt) ~ 1tsp for 3.8l of water, this will cure the fungus in 2-3 days if it is a mild case of it. Discus can handle the added salt for the time being, but remember the only way to get rid of the salt is by dilution, water changes. Okay now you have a planted tank, this will kill every snail and plants in your tank. So where i reco you have a hospital tank this is the reason why. And yes you have guessed it put the salt right in the aquarium. We do know what we are dealing with so this method is recommended only if its a mild case if its not a mild case it won't work. Just remember that salt don't cure everything. And to be honest with you, this fish should be taken away from this tank and put in qaurtine cause your other discus and prone to catching this, I am thinking you should treat them both just to be safe. Time is ticking on this matter and the life span is becoming shorter and you need to act quick.
Definitly last resort!!!!!!!!!!!!!
If all else fails, treating with oxytetracycline will kill off this infection. However, this med will also kill off your bio-filter and all bacteria in your system. It's also been known to adversely affect the digestive systems of some fish species through damaging intestinal flora.
Keep your water at the same temp about 30C will do the trick for ya just remember to watch the behaviour of the fish.
Another question are they eating okay??
Hopefully this helps for you and does the trick, remember 1 thing you want to actually treat this as it is a spreadable issue. Hope this helps, like I said I have never tried this with discus I have done salt treatments to babie fry before and also adults with success as far as some of these products go I can't warrent anything but its a really good book called Discus Fish, its an owners manul and its up to date kinda lol. Well main thing is it works.
Just remember discus are hard to adapt to a place but once they do they are usually okay and you will be able to tell signs when water changes are needed etc. Watch for the colours it will tell you alot that is going on in the water.