Though rather hardy, black moor goldfish do not have the voice to tell you when they are sick; they use more subtle ways of communicating.
A sick fish will change its colour. It can either get very dark, or in the case of the dark-coloured black moor goldfish, wash out and seem much lighter than normal.
Age and Stress
Moving to a new tank, or overcrowding can stress a black moor, causing it to fade. Age may also change its colour. Black moors turn various shades of bronze, grey, black and even orange. These fish are carp, a cold-water species. A black moor will turn orange if the water temperature in the tank is too hot.
Aggressive Tank Mates
Sharing the tank with particularly aggressive fish can also affect a black moor's colouring. It is unable to compete with fast and aggressive fish and may suffer fading due to the stress of the situation.
A black moor with white spot will have white specks over its body; these make the fish appear to be changing colours. This is not the case. In fact, white spot is a parasitic illness that should be treated with either Ickaway or Promethyasul. These medicines are available at pet stores.
A black moor fish that loses scales will appear to be changing colours. Usually this is not a cause for alarm, as sometimes a fish will rub a scale or two off on a rock or other hard surface. If many scales begin to drop off, further research needs to be done to find a reason and possible treatment.