Alot of the Brown algea is caused by a buildup of Diatoms in the water column. Diatoms are introduced by your replacement water. Brown algea is also considered a normal part of a cycle of a new tank. Most tanks go through this before growing the greens or the coraline.(red, purple, green, lavander)If you use RO or even some distalled water you do not have as bad of a brown bloom. I think most people on here have had this in their tank during the cycle.