I think something else is gholding the plants back, not the NO3, if the plants are growing very well, BGA will not infest their growing cones like that.
Fluff it off, good, do a larger water change, add ferts back.
If the BGA shows up 2 days later, fluff it off again.
If it shows up even faster, then it maybe the CO2 is a tad too low.
Be careful, add the CO2 very slowly, this will drive plant growth faster, EI assumes a non limiting nutrient level, make sure that it's all there, NO3, K, PO4, Traces, and Mg/Ca for GH.
CO2 is all that is left typically and good cleaning.
If the CO2 is good and you clean well, clean filter etc and keep after it, the BGA in those tips should go away.
BGA is not always causes by low NO3, but it certainly can be the lion's share of the causes(the real bad cases typically are, minor ones can be from low O2 production from the plants).
Regards,
Tom Barr