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Ca as PO4 remover?

     Those familiar with water treatment methods know that:
     5 Ca + 3 HPO4 + H2O --> Ca5OH(PO4)3 + 4 H
     Hydroxyapatite (Ca5OH(PO4)3) precipitates out.  The addition of 
     calcium is a good way of removing phosphate (conversely, the addition 
     of phosphate is a good way of removing calcium).  As long as calcium 
     is in excess, the amount of PO4 remaining in solution is dependent on 
     the solubility of hydroxyapatite.  
     Typically, PO4 in our aquaria is higher than our tap water, largely 
     due to the addition of fish food.  This implies that those of us with 
     ample Ca may have a natural cap on the amount of PO4 in our water (as 
     long as we do regular water changes), while those with soft water may 
     benefit from the addition of Ca.  Another benefit is 
     sedimentation--phosphate would drop out of the water column and into 
     the substrate, where it could become available to rooted plants.
     Does anyone know the solubility of hydroxyapatite, and how 
     pH-sensitive is this reaction?  If it only occurs at high pH, then it 
     isn't really feasible for aquarium use.  Would the addition of Ca be 
     as effective as a phosphate-absorbing resin?