The rock layers at the top of the Little Malawi 10g tank (my cichlid conserve) are about 4 inches from the water's surface and 6 inches from strong fluorescent bulbs. Light algae forms every few weeks on these plateaus, where they photosynthesize their little butts off, with no other plant life to compete for their minimal nutrients. I'm amazed that algae grows in an unplanted tank into which I deliver no nutrients. Just fish.