♂ When it comes to brown algae, the too much or too little light
theory is usually bogus. Here are 3 things to consider...
1. Phosphates/silicates in your tap water, or silicates from another source like
an airstone. Those bonded sand style airstones break down, and add silicates
to aquarium water. Don't use these airstones, and if you think phosphates might
be coming from tapwater when you do partial water changes, use a phosphate
remover in your filter (PhosX, PhosGuard, etc).
2. Maybe overfeeding? This would provide nutrients needed for either brown
or green algae.
3. As mentioned by someone, diatoms in a newer, cycling tank. Just keep
cleaning it off, and it may start declining on its own after a while, and then you
might begin to get green algae. If the brown algae continues, and it is not a
phosphate/silicate, or overfeeding problem, get ottos.
The light issue applies more to green algae, and that would possibly have to do
with too much light, or tank being exposed to sunlight.