Sure.Can you elaborate more on this: To reduce PO4 by 0.2ppm you need to reduce much more (say at least 0.4ppm PO4 total), since more PO4 will come out of the rocks.
Phosphate is bound to rocks and sand in addition to being in the water column. The phosphate is in balance with the substrate and water column.
When you remove phosphate from the water (water changes, GFO, lanthanum etc) phosphate from the rocks release back into the water column.
Thus, in order to remove a specific amount (ppm) of PO4, you will need to actually remove much more PO4 to get to the desired outcome.
For example: doing a 50% water change will reduce nitrate by 50%.
Doing a 50% water change will not have much effect on PO4 since more will release from the rocks.
I think the rocks are soaking up the PO4. I don’t think it’s the carbon dosing. The rocks can bind and unbind depending on their saturation point and the concentration in the water.