I don't really know how to fix your fish...but I have had an Aulunocara species do the exact same thing, with no other fish in the tank showing any symptoms. So if you do cut him up and find something, let me know. :) ...Actually, I will make an attempt to help. My former employer had an orangespot that would not do well on prepared food that the tankmates made no fuss about. He supplemented the food with crushed snails (o-spots natural food) and the fish perked up quite a bit....maybe theirs a connection.
my 2cents
Mike
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