How does the fish look in term of physical appearance? Is it thin? Is it possible to take a (head on) picture?
The reasons I asked because there are something else that can happen inside a fish. For example, did you quarantine all your fish? how about the trigger? It may have internal parasites.
Another thing that came to my mind is cyanide poison. Many of this trigger come from indo-pacific where using of cyanide is wide-spread. A cyanide poison fish can appear fine for a few months and going down hill from there.
If it was my fish I would put it in a separate tank (QT), observe and treat whatever it is.
What other fish in the same tank? Do you happen to have a lion fish in there also?
Good luck!
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