just double check your salinity with someone's refractometer to be sure it isn't way off.
Cupramine would have killed any and all bristleworms and pods and anything else that may have been alive as far as inverts in the sand bed, rock and throughout the tank provided that they were there before you used cupramine. Your spike could be coming from the dead inverts.
January to now isn't a very long time. It could have just been the luck of the draw. Unfortunately we have no idea how the fish was caught (netted, stunned with cyanide) and sometimes all of the stops along the way that a fish takes in being caught, the diver putting it a container (by itself or jammed with too many fish into too small of a space, kept at the correct temps or left in the sun, stressing in daylight or kept calm in the dark) ..... brought to a wholesaler, shipped overseas (flight ran late....48hrs or more before being taken out of the bag being seriously deprived of oxygen), over medicated in a wholesale or retail enviroment all play a part. Sometimes you can do everything right, and still end up with a dead fish.
Did you QT the fish when you brought it home?