had the same thing happen I had bought 2 percula clowns and a yellow tang and put them in my tank and the smaller of the 2 clowns died with in like 3 days then the second clown started doing the same thing "swiming near the top movin iradticly with his mouth open" did that for about 4 days then come to find out that the yellow tang had ich and the clown ended up with it. so I removed the clown and the tang and put them in a hospital tank and have been treating them for about 4 days now and he has come around alot he is more active and his mouth isnt hanging open any more.so I would keep and eye on all the fish in your tank and see if any are showing any signs of ick or any other stress related diease.