Discus not looking well

One of my large Blue Diamond discus is not looking well.
It is looking a little dark,
Is keeping to the back of the tank,
and if not eating.
It has been like that for a couple of days.
All the others in the tank are looking fine.
Can anyone suggest what may be wrong?

Regards, Irene

Need a few more details Irene. Can you give me the results of your water tests.

I need pH, GH, ammonia, nitrite and nitrate.

The size of the tank, how many fish in the tank, and how often you do water changes, and what percentage of water.

How long have you had this fish, and have you introduced any new fish into the tank recently.

Difficult to know at this stage just what might be wrong, but some of the above details will help to narrow things down.