Brown Algae needs light (your tank lights are perfect) and food
(nitrates and silicates) to survive. The cure for Brown Algae is
the same as the prevention: Starve it into oblivion. Nitrates
can be introduced into an aquarium not only as the end product
of the Nitrogen Cycling Process (ammonia/nitrite/nitrate), but
also via some brands of commercial sea salts and tap water. You
can scroll down to see methods for reducing nitrates. The
silicates the Brown Algae requires can enter your tank through
several avenues: Commercial sea salts, tap water used for top
offs and make up and the substrate you may have on the bottom of
your tank. If you did absolutely nothing to your tank, the Brown
Algae would eventually consume the silicates present and die of
starvation. Since you can't do absolutely nothing to your tank,
the source of the silicates must be identified and eliminated,
in order to get rid of the present brown algae bloom and prevent
future outbreaks. - Check the source of any builders sand, play
sand or ot