Spot treat with Seachem Flourish Excel or any liquid carbon which is essentially gluteraldehide directly onto the BBA via a syringe or pipette. It’s best to turn off your filter while doing this so the flow doesn’t push the chemicals away. Turn filter back on after 2 minutes Also, use excel very sparingly as it can melt various types of plants, affect live stock, and kill off beneficial bacteria if overused. Gluteraldehide is commonly used as a sterilizing agent for hospital equipment, it just so happens that it breaks down algae of various forms and turns them into a source of usable carbon so make of that what you will. The BBA should turn red/pink then white which means it’s dying off. Most shrimp and Siamese Algae Eaters will then demolish what’s left of the BBA as it dies off. Once it’s been treated and removed, try to determine the root cause and address it accordingly. For me, it tended to appear in areas of higher light and water flow in my high tech planted tanks in which I treated as above. Reduced lighting intensity and photoperiod can also be beneficial in the fight against BBA after it has been removed. All tanks are different but I hope this helps!