I just tested my phosphates and they are testing so low I can barely read the color showing that they are even there.
Testing for PO4
can be a bit misleading. You can get a reading of close to zero and still have algae problems. This is because there is enough PO4
in the system to feed the algae, once the algae has consumed the PO4
it may not be detectable. Basically, the algae is exporting the PO4
faster then you can test for it. Test your source water and evaluate your feeding techniques as a way to eliminate PO4
getting into the system. Stop that and you can stop the algae.