Goldfish can turn black in response to stress. Some fish are more affected by stress than others and some will show their displeasure by bleaching out in colour or turning a blackish colour. You really should not need any additives to the tank, de-chlorinator should be the only thing you need and only if you have to do a drastic water change. Also additives and large water changes can throw off the delicate bacterial balance of the filter. If the bacterial system in the filter in the tank is damaged it can cause Ammonia from fish and food waste to build up before the filter recovers (This is called a mini cycle) and this is one of the main causes of stress in fish. When he is displaying black colour you should take a water sample in a clean container to your local fish shop (or pet shop with a good fish section) and get the water tested to see if any of the parameters are off.
Last edited by cell; July 16th, 2009 at 12:15 PM.