Blackening Koi or Goldfish Skin
Written by Dr Erik Johnson

Melanophore Migration
Orange Goldfish Develops Black Patches
"I bought a fish two weeks ago, and a few days after I bought it and carried it home, it started developing dark black patches on its body, especially on it's back and sides. After a week, the patches began to fade, now it's almost all orange again! What gives?"
ANSWER:
What you were seeing is called a "MELANOPHORE MIGRATION" - It's caused by *any* chemical or traumatic irritant or injury on the surface of the fishes' bodies.
Some good & very common causes include:
#1 cause: High Ammonia concentration in water
Low pH causes irritation
Fluke parasitism causes trauma
Ich or Costia infestation causes trauma
*HOWEVER*- the black color does not come until the irritant is REMOVED and the healing process begins.
Therefore, it is safe to assume that the fish *was* in bad water or under severe stress until you bought it, and the melanophore migration came and went as usual, starts as healing starts, comes on over two weeks, resides for a week or more, and clears within three to six weeks.
"Blackening and dark spots occurring on goldfish after some trauma to the skin will usually resolve in 3-6 weeks. It's interesting to note that the black and gold Orandas, as a side note; never hold the black." Doc Johnson