The following can convert hard water into soft water :

The hardness of water is due to carbonate or sulphate of $C{a}^{2+}$ and $M{g}^{2+}$ ions. When sodium aluminum silicate is added $N{a}^{+}$ replaces $C{a}^{2+}$ and $M{g}^{2+}$ by adsorption, and hence water becomes soft.

$\mathbf{H}\mathbf{e}\mathbf{n}\mathbf{c}\mathbf{e}\mathbf{t}\mathbf{h}\mathbf{e}\mathbf{c}\mathbf{o}\mathbf{r}\mathbf{r}\mathbf{e}\mathbf{c}\mathbf{t}\mathbf{a}\mathbf{n}\mathbf{s}\mathbf{w}\mathbf{e}\mathbf{r}\mathbf{i}\mathbf{s}\mathbf{o}\mathbf{p}\mathbf{t}\mathbf{i}\mathbf{o}\mathbf{n}(\mathbf{D})$