Black marks appearing like this are usually caused by ammonia burns. Water
quality cannot be determined by simply looking at it. You need to test for
ammonia, nitrite and nitrate a.s.a.p. If you don't have the kits, buying
them is a good investment, or alternatively, take a sample of your water to
your local fish shop who will test it for you. Take a pen and paper and
write the exact results down so you can post them here as most advice given
by employees in these places is terrible (unless you are really lucky!). if
you post the results we'll try to help you.
Until then start daily 30% water changes (using dechlorinated and
temperature matched water). This will dilute any toxins in the water and
make things better for your fish.
Mel.

"Oscar" wrote in message
...
We got an aquarium 6 month ago which is populated by 8 goldfish. My wife
noticed yesterday that one of the fish, normally a golden brassy colour
has
developed 2 larger dark/black spots on the top. He seems to be happy
enough
otherwise, but my wife is worried it is some kind of disease.

The water is changed regularly, we test Ph and the aquarium is clean - so
it
looks like a healthy environment to me.

Any advice or opinion?

Cheers, Oscar