The composition of Calgon which is used as water softener is

Calgon is a trade name of a complex salt, sodium hexametaphosphate [Na(PO₃)]₆. It is used for softening hard water. 
The addition of Calgon to hard water causes the calcium and magnesium ions of hard water to displace sodium ions from the anion of Calgon.

${2\mathrm{CaCl}}_2+{\mathrm{Na}}_2\left[{\mathrm{Na}}_4{\left({\mathrm{PO}}_3\right)}_6\right]\to {\mathrm{Na}}_2\left[{\mathrm{Ca}}_2{\left({\mathrm{PO}}_3\right)}_6\right]+4\mathrm{NaCl}$

2MgSO₄+Na₂[Na₄(PO₃)₆]→Na₂[Mg₂(PO₃)₆]+2Na₂SO₄