[mr_alberta]

There are a couple possibilities:

Problem:
Chemicals in the tank (could be aerosol cleaners or anything like that).
Solution:
Water changes and Carbon

Problem:
Tank aggression/Overstocking. Mainly from either of the crabs of the Cinnamon clown.
Solution:
Removal of said items from the tank and see if the problems stop.

Problem:
Some type of internal parasite or disease since there are no external signs.
Solution:
Quaratine of fish/treatment with medication, though without knowing the actual illness it will be nearly impossible to treat.

Your tank is a 77G correct? I would do the solutions for problems 1 and 2 and see if it helps. You may also have another hitchhiker that you have not seen or even know about!