I'm going to be the dissenting voice and say that I doubt reducing your nutrients drastically will do much for the cyano.
a) it's commonly reported on fish forums that cyan thrives in ULN tanks and is often associated with organic carbon dosing.
b) mats of cyanobacteria are often the only organisms living in some of the most oligotrophic (i.e., nutrient poor to the point of being hostile to life) bodies of water on earth.
c) 'cyano' is in fact an incredibly sophisticated assemblage of heterotrophic and autotrophic organisms, including prokaryotes, dinoflagellates, diatoms, and cyanobacteria, all stratified along micro pH and oxygen gradients within the mat and fulfilling different roles in what is in fact a mini ecosystem. There is strong evidence to suggest that some assemblages can fix nitrogen directly from the atmosphere. The whole assemblage is designed to be as efficient at recycling captured nutrients as physically possible (i.e., they don't really lose any nitrogen or carbon once they catch it), and to be as efficient at scavenging nutrients such as organic carbon and nitrogen from the environment as any ecosystem can be.
I honestly think the best way to think about cyanobacteria is to equate it to an infection. It can thrive in your tank regardless of your nutrient profile, and has attributes that actually give it a competitive advantage in an extremely low nutrient environment. Once it's gotten out of control, I think hitting it with a chemical treatment is one of your best options. The goal after it's gone is to try and encourage the kind of microscopic competitive regime that favours forms of life other than cyano, which, given the fact that it's such a common and unending problem in the aquarium trade, seems to be incredibly difficult to do.
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