Phosphate tests can be tricky. A lot of the times they aren't that accurate. A "zero" reading just means that phosphate is lower than the tests range. Unless of coarse you are using a colorimeter or a digital titrator.
Another way to look at it is you have algae in the tank taking it out as fast as it gets put in. You aren't going to record nutrient levels until there is an imbalance.
I had the same problem with DT's, try feeding half as much
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