I find these Cyanobacteria blooms come and go. Feeding your fish flake or other foods can raise your phosphates by point four very quickly and then drop back to zero. Point four is loads and can be fuel for these blooms, but ultimately your tank can read zero at all other times. Reduced feeding, macro algae and plenty of sand sifting life forms should take care of your problem. But I'm only speaking from experience, as I'm sure you know, what's true for one tank might not be true for all.