Rocks bind and unbind po4. They do not leach or absorb it. There are rules with binding and unbinding, so once you understand them you can solve the problem. The rock does not just soak up po4 at will and then let it go whenever it wants. You have to raise the water column level for the rocks/sand to BIND more and then lower the water column level for the rocks and sand to UNBIND it.
There could be 500x, or more, po4 bound up in the rocks and sand than there is in the water column. Nobody knows since surface area is a huge part of this and all aragonite has different surface area structures.
There could be 500x, or more, po4 bound up in the rocks and sand than there is in the water column. Nobody knows since surface area is a huge part of this and all aragonite has different surface area structures.