First off, cyano grows like mad in the ocean where the light bulbs don't need a changin', and the Nitrate and Phosphate levels are tremendously low....having said that.....think about what that leaves.....low flow.....cyano doesn't grow on the reefs where the waves are constantly churning the water....is grows in lagoonal areas with very low water turnover.
One other thing to note, carbon dosing is also not the only way to maintain an unmeasurable level of phosphate and nitrate...the same result can be had with a well maintained fuge, using a Sulfur DeN., great husbandry skills, deep sand bed, kick a$$ skimmer, etc....