Why is my fish turning black?

I need some help!!!
I have a comet goldfish and it is turning black, at first it was just a bit on the back of it but its gotten bigger and bigger and now it is almost completley black.
What is happening????
I had another fish before and the same thing happened but it died.
Is there something wrong with my water or anything?
HELP!!!!!!

 

How big is the goldfish?
What kind of goldfish (fancy, common/comet, shubunkin?)
How big is the tank?
What are the readings for ammonia, nitrIte, and nitrAte?
What other fish are in the tank and how big are they?

Can you post a picture?

-Chelle

 

the fish is a small comet
the tank is 24 and a half cm x 18cm
i don't test the water so i don't know the readings and
i have two other fish they are really small zebra fish
sorry i don't have a pic of it because it just died and it is gone but it is basically large spotted areas of black

Thanks heaps

 

My guess is ammonia burns. That tank was WAY too small for a goldfish (probably around 3 - 5 gallons). You really need to get tests for ammonia, nitrIte, and nitrAte. You're in danger of losing your other fish if your ammonia is high enough to cause burns on the goldfish.

-Chelle

 

Ok then ill do that
Thanks so much for helping me