I think algae start growing preferentially on unhealthy leaves. In my tank I see much more BBA, when I have it, on older leaves that are not actively participating in the growth process - in other words, they are on the virge of dying. Once any algae gets started, higher CO2 concentrations are as welcome to the algae as to the plant, so I have had lots of BBA, again, on filter return pipes, on areas right in the flow from CO2 reactors, etc. The secret seems to be avoiding the algae ever getting started, and avoiding any unhealthy plants in the tank.