Goldfishes Tail Turning Black

[rubberdoofa]

I had a major tank disaster some time ago and lost one goldfish when i noticed another goldie with what looked like fin rot on his tail( white cottonwool fluffy stuff) i dosed the tank with salt gradualy did water changes etc but he wasnt getting better i baught some fin rot stuff and with in hours (almost) it grew better and a day or 2 later its gone but he was left with a ragged tail now i see its turning black at the raggedy bits on the ends!! hes still a lively fish eats well and gets on with the other 3, i know im over crowded that along with a lack of a proper cycle caused the initial problem can any one tell me why his tail is going black?...it wont drop off will it
they are in a jewel R96 with standard jewel filter and a fluval 2

 

[rubberdoofa]

I had a major tank disaster some time ago and lost one goldfish when i noticed another goldie with what looked like fin rot on his tail( white cottonwool fluffy stuff) i dosed the tank with salt gradualy did water changes etc but he wasnt getting better i baught some fin rot stuff and with in hours (almost) it grew better and a day or 2 later its gone but he was left with a ragged tail now i see its turning black at the raggedy bits on the ends!! hes still a lively fish eats well and gets on with the other 3, i know im over crowded that along with a lack of a proper cycle caused the initial problem can any one tell me why his tail is going black?...it wont drop off will it
they are in a jewel R96 with standard jewel filter and a fluval 2

goldfish is now fine again his tail has recoverd and is still attatched to his body and all is looking well