Black Goldfish?

Question
I had a gold-colored goldfish that died not too long ago, but unsure why. It may have had a hard time getting out of a little bridge I but at the bottom for my ghost shrimps. After that, my white-colored goldfish started losing its fins and was always shaking. I wasn't sure if it was going to die or not. I really didn't know what to do. After a while, it started turning black. It's fins grew back except for the back tail. What's wrong with it? It's been like that for almost 2 weeks now. It hasn't infects my other two goldfishes which are one gold and one white.

Answer
Hi Jennifer
What size is the tank?  

It's recommended to have at least 15 gallons for one goldfish.  And, that's for the fancy varieties, commons and comets will need a larger tank then that.  Usually when a goldfish turns black like that, it's a sign of ammonia burns-and the black is actually the healing stage.  Fin/tail rot is also a sign of poor water quality.  So,  I would suggest checking and making sure the ammonia and nitrites are at 0 ppm, and nitrates under 20ppm.  Though, it may have gone back to normal, but that's usually what those symptoms mean.  

Christy