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Appearance
Brown algae range in color from light tan to dark brown, and generally form as a coating of slime over aquarium rocks, structures, pipes and glass. The most common types are kelp, which if left alone can grow a few hundred feet long. Chances are, the type in your tank won't grow into something that grand.
Causes
An excessive amount of nitrates -- or fish waste -- in your saltwater is the greatest contributor to an overabundance of brown algae. The presence of silicates from tap water, something that aquarium water conditioner can reduce, also can facilitate a higher-than-normal level of brown algae. Another possible cause is overlighting your tank. According to Foster and Smith Aquatics, however, the leading cause is likely to be inadequate cleanings and overfeeding of your fish, which leads to more waste.
Eradication
Reduce the amount of food you feed your fish everyday. This will reduce the nitrates in your water. Clean your aquarium with a clean rag and your rocks under running water at the sink. Use a water-changing tube to clean your gravel as you change out at least one-third of your saltwater about once a week. Replace dirty water with conditioned, pH-balanced and properly salted water. Changing too much water at once can upset the fine balance of beneficial bacteria; changing too little won't reduce the bad types like brown algae.
Other Problems
Continued problems with brown algae might signify a problem with your filtration equipment. Make sure that the pump and filter are processing tank water adequately, that your charcoal filtration system is fresh and filled to the top. Another problem can occur when brown algae form all over the intake ports of your filter, reducing the flow of dirty water out of your tank. Clean all equipment hanging on the sides of your tank as part of your cleaning regimen.