I have had problems with that, and when it is in those dark spot form it is impossible to get off. In each case, the leaf eventually died even when I spot treated with Excel or H2O2. I would try to cut of the very damaged leaves and then spot treat the ones that aren't as over taken yet.
Do you have it growing on anything else in the tank? If you see it starting in places, treat it early and treat it often with Excel spot dosing.