When I had the rusty brown algae problem I found that adding a PH buffer to the tank helped a lot. I also poured the buffer I mixed directly into the tank and over the rocks which were the worst and it seemed to kill it. I don't know if this was the biggest factor or not. I looked at it like chicken soup for a cold--may not help but it won't hurt either. I may have just gotten lucky and I am sure that someone out there will correct me. However, right or wrong it helped in my tank.
Greg