Planted Tank Guru
Join Date: Jul 2010
Location: British Columbia, Canada
I know for sure that hydrogen peroxide does an excellent job of getting rid of blue-green alga (cyanobacteria), and I have read that it will do a good job of removing black beard algae too, although I haven't tried it myself.
Try dosing the BBA with full strength H202, at close range, using a plastic syringe, or test kit pipette, with the filter(s) off. Leave the filter off for about a half hour after dosing, so that the water flow doesn't flush the H202 off the BBA you've just dosed.
Not sure what you need to do to remedy the root cause so it doesn't return, but try improving/increasing the water flow throughout the tank, using a power bar, or water circulation pump of suitable size for the tank.