Have you tested your nitrate levels?
I had a bout of BGA a few weeks back and I tested my nitrate levels and found them to be at 0. I started making certain that my levels were 5 to 10ppm and treated the tank with erythromycin and vacuumed out what I could. I licked it in 2 days but dosed for 4 days for good measure, and it has not returned since.
--- Dave ---