I FORGOT......WELCOME TO FISHLORE! 
I really hope we can help out.....
OK, I read your tank profile....THE very first thing you have to do is test your tank water....You stated you don't know about the nitrogen cycle.....PLEASE read up on this....we have a very detailed article you can access by going to the top of this page under "ARTICLES". click on the link to the nitrogen cycle.
To keep track of your water conditions in your tank, you have to test your water....every week, and chart the results...This is the only way you will truly know how often and how much water to change.
Most here at fishlore suggest you use the API Master test kit for freshwater (liquid)...It gives all three of the major tests for keeping a healthy tank... Ammonia, Nitrite, and Nitrate.....
You should have 0 for ammonia and nitrite in a completely cycled tank....the only time this would fluctuate if if you do extreme tank cleaning, or change filter material...always test after either.....
Nitrates are "safe" if under 20.....for most fish. Discus however need as I stated above PRISTINE water conditions between 5 & 10,
NO HIGHER THAN 10, which with the tank stocked as you stated, I can assume might have high nitrates, that would definitely cause the condition your discus is experiencing.....You need to try to get those nitrates down, and the only way to do this is through lots of water changes.....every day as much as 2
x 25%! You will have to keep this up until the proper nitrate reading is achieved, then as often as necessary to maintain it.
If you haven't been testing for nitrates, it might shock you what the reading is...
What water changing schedule are you currently using? Do you vacuum your tank?