Why does your fish's fins turn black?

Goldfish do change colour as they get older and also due to enviromental reasons.

Good quality food, good lighting and good water quality mean nice brightly coloured fish. Black is a normal colour for a goldfish.

The opposite leads to them fading or loosing colour.

My fish used to have black flecks, dad neglected him when I moved away and he lost all his colour and was a pale orange. Now I have him he's nice and healthy, turning bright orange again and gaining his black flecks back :)

If your fish is sick he will probably be at the bottom of the tank alot with his fins clamped to his sides and will generally look dull. :)