Having a high concentration of CO2 in the water is no guarantee of having no BBA. In fact BBA likes the growing conditions on the spray bar through which the CO2 enriched water enters the tank. I think high CO2 levels are a help in preventing BBA, but keeping dead leaves removed, keeping a good level of substrate cleanliness, and quickly removing any BBA that does show up is also very important. I say this being one who is also frustrated that BBA continues to show up in my tank apparently just to teach me humility.