Did you quarantine the new fish? Have you added anything else (other fish, corals, etc.)? How did you add the new fish?Coloration in many fish can change depending on attitude, body conditions, health, etc. The dark spots are not necessarily anything in particular that you can point out as a symptom.
I understand it can be very upsetting, but you need to understand it is extraordinarily difficult to diagnose illness on a verbal account. While I dont feel you were necessarily trying to do so, I dont feel you can blame the club and its members for not coming up with a solution.
As for my opinion, I cannot say definitively, as I have not seen the fish, now or before. The symptoms of lethargy, listlessness, and poor appetite sound to me to be leaning towards brook. I would just give it good clean water and try to keep feeding it. It sounds as if this fish is very weakened already, so treatments for some unknown pathogen can easily do it in. If you can see no other symptoms aside from dark spots, I would just try to provide optimum conditions.
