Re: Calcium etc.

CaCO3 is indeed almost insoluble in water. To dissolve, bubble CO2 into 
your tank as CaCO3 + H2CO3 = Ca(HCO3)2. If you want to add just Ca, then 
use CaCl2 instead of calcium carbonate ... it is much more soluble and 
wont affect your alkalinity. High Ca levels in your tank will precipitate 
PO4, as will ferric iron ... in fact iron phosphate precipitates are a 
major part of the P cycle in freshwater systems ... as they enter the 
sediment the iron is converted to ferrous iron and the P is released back 
into the water. 
I have never read that high pH seriously reduces the chelating capacity 
and stability of EDTA ... I would be interested in a reference. I also 
doubt that Ca could displace Fe since the FeEDTA complex has a formation 
constant 15 orders of magnitude greater than the CaEDTA complex ... thats 
a 1 with 15 zeros behind it.