BGA prefers stagnate water. If you don't have a really bad case, then vacuum as much as possible, don't try to remove it by hand, lots will break off and the goal is to remove as much as possible without causing a disturbance. After vacuuming aim the strongest water current you have directly at the remaining BGA. BGA should continue to shrink and eventually disappear completely. However, if it's a really bad case you may have to resort to treating with erythromycin.