Brown Algae are usually diatoms which thrive in low-light situations or in tanks with high silica content. I don't think I've ever had a tank where there weren't at least some patches of the stuff. Very few algae-eating fish will even touch the diatoms, though I found that my Farlowella cat would browse on them if he was hungry. Fortunately the stuff is relatively easy to wipe off the glass, though it can sometimes be difficult to get rid of on plastic plants. Some people say that water changes will get rid of them but I've never had any luck doing it that way. Higher light levels will do it sometimes but then you have a problem with other types of algae so it becomes a trade-off.
