CaPO3 solubility

Some questions for the chemists and botanists on the APD.

How soluble is CaPO3? If an aquarium has significant amounts of
Ca ions will these precipitate the majority of phosphates? How
much could stay dissolved in ppm of phosphate assuming a surplus
of Ca ions (at ph of 6.8 ot 7)? If phosphate is in surplus, how
much Ca will be dissolved?

If most of the phosphorus ends up as CaPO3 or FePO3 (precipitate), 
is this a usable form for plants? Papers I read refer to DRP or 
dissolved reactive phosphorus so presumably some forms of phosphorus 
are not reactive or available to plants.