The biggest cause of brown algae outbreaks is often silicate.
It can make its way into the tank of the unsuspecting aquarist via low quality salt mixes, unfiltered tap water, additives, and even substrates!
Even the most diligent algae eaters will have a hard time ridding your tank of brown algae if the level of silicates isn’t drastically reduced. The diatoms will simply reappear in batches that your algae eaters cannot keep up with, and you’ll find yourself having to clean your aquarium glass, equipment, and plants often to keep your aquarium moss-free.
The second biggest cause of brown algae outbreaks is excess nitrate levels. Beneficial bacteria that break down waste in the aquarium are able to convert the highly toxic ammonia to a less toxic nitrite, then from toxic nitrite to the least toxic nitrate.
Unfortunately, however, the bacteria that can convert nitrate to hydrogen sulfide gas needs an anaerobic environment, which is difficult to produce in most aquariums without specialized, expensive equipment.
Plants are able to remove some nitrates, but in tanks that are sparsely planted, nitrate levels can rise if water changes aren’t consistently performed.