On Wed, 24 Sep 2003 13:25:14 +0100, "Mel" wrote:

The black marks are ammonia or nitrite burns.

Hi,

Just started to read this thread. From everything I have read
the black comes as they are healing. You said one of the fish
had them when you bought it and then they went away. That
is because he/she completely healed. If they come back that
is because they have been burned again and are once again
in the healing process.

No matter what the problem with a fish. The very first thing to
do is water change, water change and water change. This
seems to be the root of many many problems and doing it
does not harm the fish.

Good luck with your guys.

Azul