blue discus turning dark

Question
one of my blue discus turned black.....otherwise it is eating, & acting normal.....no other problems.....
Help

Answer
Hi Muriel,
This is not a good sign.  It is usually caused by water conditions or stress.  Check your water, make sure it is acidic, free of ammonia, nitrites, and nitrates.  Watch to see if anything is stressing him in your tank.  Without seeing the fish, it is hard to say what he may have, but my first action would be a water change, and see if anything is bothering him in the tank.  
Lynda