Cannot get rid of cyano!

[beefORchicken]

cyano uses nitrogen fixation ->grows without nitrates and uses nitrogen gas in the air, but it still needs a source of phosphate.
large W/Cs did nothing to cyano for me
extra GFO worked, but it took a long time and was a gradual change with less growing back each time it was removed
(GFO also removes silicates which can contribute to cyano)