High phosphates are the primary cause, but high nitrates also contribute. Cyanobacteria do not need much light.
Now let's mess with the high side:
I dose 30-45ppm of NO3 and 15 ppm of PO4 as KNO3 and KH2PO4 weekly to this tank, see any evidence of BGA? Any algae for that matter?
Tank also is dosed 2 ppm of Fe as a proxy for traces......as CMS +B.
If what you stated is true, then how come I do not have algae or BGA?
If could be many many things(this we cannot ever be 100% certain).......but those 2(PO4 and NO3 being high) are not it(this we can be certain).
I think 15ppm and 30-45ppm a week of those would qualify for being on the "high" side.
Maybe I just got lucky this one time? I have about 50 tanks going back 20 years with similar results: