Now its always hard to tell by ones picture. I'm sorry to tell you.. it looks like Brooklynella. It is a parasite disease that happens to Clownfish. Its nasty and most treatments for it are ineffective. The fish will rot away and produce mucus, slime over its body. It can also display as a white slimy film.
For more info.. check out the disease section on canreef,
http://www.canreef.com/vbulletin/showthread.php?t=22013
that has some great links to read on this. Or google "Brooklynella"
If your sure its been injured by the overflow, you can place the clownfish in a QT, and use antibiotic or Meleflex to help repair fins.
Hope this helps.