Black Mold in fish tank, help!

[Mike]

I have had my 46 gallon tank running now for several months with 6
medium sized gold fish in (wanting to add more soon). Last week we
started to see a black mold growing on the glass and on the rocks.
Now its gotten a little worse. The water is testing fine and the fish
are healthy as can be, but the rocks (bottom rocks and large display
rocks) are showing a lot of black mold on them. My alge treatment
didn't do anything to it. Any ideas guys & gals?

Thanks in advance,
Mike

[_]

This is for Marine fish.