Thanks for your input.It took several *weeks* of heavy, heavy lanthanum dosing to get my phosphate down from .65-.70. One day, all of a sudden, I had to cut my lanthanum dose down to about 5% of what it was. I can only attribute this to the rocks or substrate “leeching” so there may be truth to that but it takes some time to catch up and then stabilize at a much lower level of dosing (or none). Like longer than a few days - not saying I recommend doing this, just pointing it out.
I feed an embarrassing amount of food (frozen, nori, and pellets) and still am maintaining the very low level of lanthanum to keep po4 around .15-.2. I suspect your po4 is not being stubborn due to feedings but rather this leeching phenomenon.
I will try lanthanum again + macro algae in sump to get 0.1 phosphate - still debating ammonia vs. nitrate dosing for 2-5 ppm nitrate