Phosphate tests are only semi-good... just because it reads no phosphates, doesn't mean there not any. Phosphate bonds with calcium ions (not testable in water column) which is why the cyano tends to coat the SB, LR, and coral skeletons(all of a calcarious base). IMO, syphon the cyano from where it is coating during a waterchange so as not to allow it bachk into the water column, but also not to disturb the sandbed over a 1/2". Good luck.