sailfin tang laying on bottom of tank

[Dert42]

3 year old red sea sailfin tang
been nice and healthy.
the last few days his cheeks sunk in a bit
he's still plump, he was eating yesterday, swimming this morning
now he's laying on his side on the bottom of the tank breathing semi-heavy...
i'm thinking a parasite?
the only thing i can think to is a quick fresh water dip...

any ideas?

 

[pleasants9]

have you added any new fish or inverts to the tank?