I remove the most heavily-infested leaves and dose Excel at 2 to 3 times the recommended strength, for 3 to 5 days.

Healthy plant growth supposedly prevents or discourages it.

It might also attack individual leaves that are in a weakened state. I recently transplanted some Sagittaria subulata that had developed abnormally long leaves because of the conditions in which they were kept. In the new tank the old leaves weakened and were replaced by new, normal length leaves. The old leaves developed BBA; the newer ones have not.

Bill