Your ferts are fine, if anything, they are on the heavy side. There are several ways to get BGA, and low nitrates is not the only cause. Wanna know how I know? In my cichlid tank, I have BGA, and I assure you, nitrates aren't low.
BGA can also be caused by eutrophic water, or nutrient saturated water. There are several ways to avoid this. First of all, keep a clean tank. Large water changes weekly are required. Good circulation to avoid dead spots, and removal of excess mulm and debris will also help conditions. You may want to monitor your nitrate and phosphate levels to see where you are at the beginning and end of the week and adjust dosing as necessary. The EI method assumes maximum plant growth, and if you don't have it, you will be left with eutrophic conditions, hence BGA.
You will need to kill the BGA off. Use Erythromycin and kill it; be aggressive and follow through with the dosing. It won't kill off your biological filter (I tested that long ago). Get that nasty crap out of your tank and work on your water conditions. Water changes, 40-60% every week!