What does it mean if your fish is turning black?

Probably its water is rotten and putrid. Please do a water change ASAP and then use the following info. The fundamental rules for keeping any kind of fish are :- 1 inch of fish needs a minimum of 1 gallon of water. (more is much better) :- Every tank needs a permanently running cycled filter (it neutralises ammonia) :- Every tank needs at least 50% of its water to be replaced every week. (Keeps excess nitrate levels low) If you keep to the above rules your fish may stand a chance of survival. If you don't keep them I can guarantee that you will constantly have problems with your fishes health.