OK - seeing as the tank is well established and stable, there are no other corals, and one other very docile fish inhabiting the tank - I'm going to go with the probability that your clown nipped a bristle worm or some other type of stinging/toxic invert in your tank. She/he looks very healthy and plump (other than the mouth being open) which is a good thing - if the clown is not eating as well for a couple more days before the swelling goes down, it will be OK. I wouldn't freak out and start putting a load of medications in there - maybe a vitamin soak for the food (like Selcon) and possibly some type of antibiotic to prevent a secondary infection in case whatever your fish came in contact with resulted in a blister inside the mouth. The food should be soft and chopped fine so that your clown can scoop it up into it's mouth without having to use it's jaws for the time being. If the clown seems to be getting worse, stops eating altogether, or if you notice anything else that looks odd, let us know! I would think from what you've described that the clown should pull through this, and hopefully within about a week or slightly more, will be back to normal. Please keep us posted on how it's doing?
W.
