Been fighting cyanobacteria myself lately. Even though my phosphate kit read 0ppm I decided to run a phosphate binder as a last resort. The growth of the cyano has been stopped and my cerith snails (only have two) are GOBBLING it up. My nassarius, turbos, and astreas wont touch the stuff but the ceriths love it. My hermits eat a little of it too, but their activity pales in comparison to the ceriths.
Long story short, even if your test kit reads 0 phosphates, dont believe it and run some binder in there. If you dont have cartridges for it, buy some, wrap it up in some women's nylon knee-highs and stick it in a bubble wall in your sump (thats what I had to do).