Calgon is used for water treatment.

$\rightarrow 2^{\text {nd }}$ most abundant element is "Si" and it is not present in calgon

$\mathrm{Na}_{6} \mathrm{P}_{6} \mathrm{O}_{18}=($ Graham's salt $)$ (Sodium hexametaphosphate)

$\rightarrow$ It exist in polymeric form as $\left(\mathrm{NaPO}_{3}\right)_{6}$ and water soluble compound

$\rightarrow$ It removes $\mathrm{Ca}^{2+}$ in soluble ion but not by precipitation