Cardinal is not eating

[donnaokla]

I converted my 55 gal tank to salt water about 7wks ago. About a week ago I added a Cardinal. He is not eating and is breathing a little heavy. No other symptoms. Could he have internal parasites or what could be wrong? I have 3 Dempsolfish and a maroon clown fish in with it. No live rock or sand.
also, I have a 10 gal aquarium, can I set it up as a quartine tank? or is it to small.

 

[donnaokla]

The other fish are not bothering him. And the water tests fine. I even had 2 different pet stores test it for me.