Excel is excellent for killing minor outbreaks of BBA. The stuff turns reddish or purple, then white, as it dies. And many fish eat the dead BBA. But, I have never had success killing large outbreaks of BBA. For those I always just remove the infested plants, remove and use bleach water on hardscape or other non-living materials in the tank. Then, with only a small infestation left in the tank, Excel works. Correcting the cause of the BBA can keep it from coming back, which, for me, is almost always an over grown mass of plants in the tank, inhibiting the free circulation of the water.