I have a 55 FW community tank, heavily planted. Substrate is Flourite and black gravel, with some slightly larger agates, interesting small rocks thrown in for interest. Why is it that brown/green algae grows and sticks only to the tiny pebbles, agates, etc. in my substrate? It has gotten crazy. I don't get much algae growth on glass, plants, ornaments, equipment, just the little rocks. I feel like taking a tiny rake and removing each one (THAT would take all day and night!). I dose regularly with Excel (a little on the light side), feed Flourish and Flouris trace at recommended intervals, use Seachem Neutral Regulator when making water changes (30% weekly). Nitrates hover at 5-7 ppm. Could one of these chemicals encourage the growth of the rock algae - OR JUST NOT WORRY ABOUT IT? The fish don't seem to mind. I think it's just ugly. How many of you think I'm being a little obsessive-compulsive? 
