There is no algae that causes me more grief than black beard algae aka BBA. I've researched and read many opinions on what causes BBA, but I've yet to find a clear answer. In fact, the answers are often conflicting citing variables like flow, nutrients, lights, CO2 i.e. basically everything common to a planted tank setup.
My own experience with BBA shows that:
The only success I've had is with hydrogen peroxide particularly when I use it for spot treatments. I have two methods for spot treating. Regardless of the approach I use about 1 ml of hydrogen peroxide per 2 gallons of water.
At the moment I'm able to limit the BBA growth with hydrogen peroxide, but if things get worse I may consider these other approaches:
Check out these related articles: