|My goldfish is developing black patches on its face and fins. What's this all about? I had bad water last week.
Another person: I recently went on vacation and left my goldfish with a 7 day feeder block in a 10 gallon tank with the fliter and air stones running. When I came back, my fan tail goldfish had mysteriously changed from clear to dark, almost black. My other goldfish are fine, just this one! Should I be concerned about my little fish friend?
This is a melanophore migration. It's caused by damage to the epidermis, which as it heals, begins to appear black in color.
It should be differentiated from Shimi (Look it up!)
The second case listed above is especially interesting. Remember, the blackening doesn't occur until the insult to the skin has been removed. Then healing can start and you may see the blackening. IN this second case, it's likely that when this person is home, the fish are over fed, and Ammonia is a chronic irritant to the skin. When she goes away, a feeder block is used which normally delivers a piteous amount of food. With the system "under fed" the the owner's absence the Ammonia is reduced and the fish starts to heal.