Brown algae is often symptomatic of newly set-up tanks. Before your bio-filter is completely established you may experience low O2
levels and/or elevated NO3
levels which can lead to the formation of brown algae. Once your bio-filter matures it is able to provide plenty of O2
and has good colonies of nitrifying bacteria to deal with the nitrates produced by waste product breakdown. Low light levels can also increase your chances of brown algae. How many watts per gallon are you running and what size is your tank?