How is the pod population in your tank? Can you see some on the rocks or on the sand/gravel?
What other fish are in the tank.
I'm thinking that this fish was captured using cyanide. What are the key symptoms of cyanide poisoning? Also is there anyway to treat the fish if it is cyanide?
Mandarins are usually rather caught with mini crossbows and little arrows than with cyanide, even in places like the Philippines. But some places may still catch them with cyanide or collect them as collateral damage when catching other fish with cyanide.
It is really hard to tell as the same symptoms also show with diseases, intestinal infections and parasites.
Some got a dose just enough to cause liver damage. Many usually eat well and always have full bellies but still loose weight. But this could also be symptoms of an intestinal infection with bacteria or worms.
Other fish that got a higher dose or are more sensitive may be apathetic and show no interest in anything. They usually waste away quickly.
There are also some that will survive but may be infertile or have other long term health issue.
Others that just got a mild dose may just do fine.
According to Kordon treatment could be done with
Methylene Blue:
"...
The properties of the drug as an oxygen transporter (it converts methemoglobin to the normal oxygen carrying component of fish blood, hemoglobin) allow it to be used in the treatment of known cyanide and nitrite poisoning of aquarium fishes. It has been suggested that newly arrived marine fish placed in Methylene Blue can have their survival rate increased as Methylene Blue aids in the reversal of nitrite and/or cyanide poisoning.
..."
I seriously doubt that it will be able to reverse liver damage. Also it probably would need to be administered right after the fish was caught with cyanide.
Lastly, I'm not sure Methylene Blue is without its own harm to reef fish.
