Your phosphate levels are high enough to sustain cyanobacteria growth. Because you are dealing with a tiny amount of phosphate, use some GFO in a mesh bag placed in your sump. Try 1 teaspoon of GFO to start and siphon out as much of the cyano when doing a water change. If the GFO is removing an adequate amount of phosphate, the cyanobacteria will not regenerate as quickly or may not grow back at all. Your RO water is in very good shape so the phosphate is coming in through food. I get blooms of bluegreen cyanos after feeding Rod's food......Jim Z.
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