It's caused from an unbalance of co2
. Reducing you lights helps because your plants can't eat as much co2
. The best option is to get a constant source of co2
. Either injection or liquid. Flourish excel works amazing for bba. Everyone told me to overdose but I wasn't going to do it in my discus tank and it also says right on the bottle don't overdose.
Anyways it works great and the bba will turn a redish colour before it dies off.