There are several possibilities here. By removing the powerhead, you lowered the dissolved Oxygen in the tank to a level that your fish "air-gulped". I know that isn't a real scientific term but it's what I use to describe it. If they get too much air in their gut, it can override their swim bladder and make them involuntarily float. (That's why it's not a good idea to just throw come flake food on the top of the water....if you use flakes, pinch it in your fingers and hold it under water for a few seconds to make sure the flakes sink).
With all of the above said, do I think that's what happened? Nope. Here's why. I clued in on two key words in your question. CLOWNFISH AND MORNING. Clownfish sometimes sleep at the top of the tank just like you described. Corky (the clown...my true perc) did for ages until I bought a Sarcophyton that he hosted in. I even consulted with the fish master....Terry Bartleme. If you need a good source for fish diseases, visit his website
http://www.marineaquariumadvice.com/ and read his articles on fish stress, diseases, etc. However, just to make you feel more comfortable, I went searching on his personal forum for sleeping clownfish as I knew I would find one. Here's your link
http://www.marineaquariumadvice.com/...pic.php?t=2566