What does it mean when the tip of a fish's tail has black tinge?

Question

Assuming that was not the state it was in when bought.

Answer 1

That's usually a sign of a previous ammonia burn beginning to heal. It may get a bit larger and more distinct before starting to fade within a week or 2. If it spreads significantly you may be dealing with a disease instead, but most likely it's a sign of healing damage.

MM

Answer 2

The fish has been exposed to high ammonia and the blackening is a sign the fish is healing. Or its a ph burn. THe ph in your tank is different than where the fish came from. Either way blackening of the fins is nothing to worry about. It will go away in a few days. Watch the bahavior of the fish. Any signs of stess and it could be sick. But if its booking around all day and its fins are up and out no worries. You can tell the fish is sick simply by the way it acts. Goldfish will let you know usually.