percula clown cannot close mouth

[yoo]

please can anyone advise me what to do. I have a reef tank with 2 percula clowns in. For the last few hours one of them has its mouth wide open and cannot close it. It is open so wide you can see right down inside it. what can i do as it will die if it cannot eat.
thanks

 

[melosu58]

What happens when you feed him? Welcome to AA. He could be breathing hard. What are your water parameters. Check them imediately.