As mentioned earlier, BGA is closely associated with low levels of nitrate in planted tanks. I've had it before and it disappeared with water changes, increased water flow and nitrate dosing (coupled with manual clean up). I just wanted to add, hobbyist kits aren't very accurate. Have you calibrated your test kits with a known solution? Even if your test results are showing 5 to 10 ppm, the actual nitrate levels in your tank could easily be virtually 0. And with a BGA problem, I would be quite confident your levels are too low. Flourish Nitrogen will work fine, though as others have stated, in the long run, you may find other sources of nitrogen, already mentioned, much more economical.