But they are more active at night. So, you wouldn't see it. But, it may not be them, that was just an idea. Since the gourami has been in there for awhile, your params are good, and no other ifsh is showing signs of illness, I personally think bullying is my best guess. Like I said, pearls can be shy, so chasing would be enough to send one hiding. Also, you said just one fin was clamped, and that sounds more an injury to me than anything else. You said you saw it in the morning, so whatever happened (if anything did), obviously happened when you weren't watching.