Diatoms thrive in bad light, but are readily out competed by higher plants and algae. I know Cichlids like to uproot plants, but you could consider Java Moss or Java Ferns which will attach themselves directly to rocks and ornaments (Just tie them on with fishing line, or even use cotton string. By the time it rots away they'll be well rooted) and my friends cichlids don't seem to bother it. As an added advantage, you can regularly trim it to eliminate nitrates from your tank (Hell, you can even sell excess plants on ebay)
