Tail Turning Black?

[starscream]

I have a goldfish in a 55 gal tank and he has had his tail nipped at quite a bit and for some reason his tail and the tail area are starting to turn black. What's happing to him?

 

[loraxchick]

It could be bacterial infection from the nipping or ammonia Burns. What are you what are your water readings for ammonia and nitrites? What tankmates. Can you post a pic?
Cheers