You are fighting the same thing as me. Take a look at my post called algae farm. I'm trying to figure this one out as well. Bga isn't exactly an algae it's cyanobacteria, and from what I've gathered on the internet, it's tough to beat
I'm trying to understand bga. Makes no sense for it to be growing there. Everything I read on the issue say low flow is a factor. I think bga needs to seriously be relooked at. Are you sure it's bga. Can you get a picture of it?