clownfish mouth won't close

[Seamonkey]

Hi all,
One of my 2 clowns seems to be off food and has a permanent open mouth, and it looks slightly swollen. It is the larger of the pair. It swims fine, but seems to be somewhat weak. (rests very quietly in the anemone). Respirations seem normal, and the other 3 fish and corals are fine. I'm not sure how long this has been going on, as my host anenome moved under a overhang and they tend to be out of sight more. If anyone can help I would appreciate it.
Toni