I just went through a velvet outbreak and lost 4 fish very fast. The rapid breathing is a sign.
How are your water parameters, Ammonia, nitrite levels, perhaps there is a water quality issue.
Did you quarantine him before adding?
Look at him in the predawn morning thats when a parasite is the most visible, at least it was on mine. If you see any white specs or dusting, be prepared to treat with copper in a separate QT
. If you treat in the main, you will not be able to add inverts without getting rid of you substate and anything porous.