how long have you been running Foz down ? It may not take out all of you PO4 immediately. Generally these sorts of things take time. How much time depends on how much PO4 is trapped in your rocks. TAke it out to preassure will only clean what's on the surface but won't resolve the issue of what's trapped inside the rocks, ie, P04. Continue your use of Foz down and give it some time to settle out.
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