Oranda- fin seems to have turned a bit black.

[wizz007]

I've got 4 orandas and they've been really healthy, but now one of them seems to have some black on one of its smaller fins. Its almsot as if its withered slightly and he can't seem to use it properly. The other weird thing is that the other fish are now chasing him around the tank.

Is this some kind of fin rot and should I separate him from the others?

Any suggestions much appreciated!

thanks.

[kevkoi]

The black on the fin sound somewhat like and ammonia burn. Some fish are more prone to a small ammonia spike than others. Did you just recently wash out the filter? If u did, that is what might have caused the ammonia spike. That said, the fin turning black is a sign that it is healing.