To increase significantly the concentration of free ${\mathrm{Zn}}^{2+}$ ion in a solution of the complex ion 

${\left[\mathrm{Zn}{\left({\mathrm{NH}}_3\right)}_4\right]}^{2+}$.

${\mathrm{Zn}}^{2+}\left(aq\right)+4{\mathrm{NH}}_3\left(aq\right)\rightleftharpoons {\left[\mathrm{Zn}{\left({\mathrm{NH}}_3\right)}_4\right]}^{2+}\left(aq\right)$

Add to the solution some:

The following reaction obtained when the zinc ion is treated with the ammonia.

${\mathrm{zn}}^{2+}\left(\mathrm{aq}\right)+4{\mathrm{NH}}_3\rightleftharpoons {\left[\mathrm{zn}{\left({\mathrm{NH}}_3\right)}_4\right]}^{2+}\left(a_q\right)$

Hence, to increase significantly the concentration of ${\mathrm{zn}}^{2+}$ion in a solution of the complex ion $\left.{\left[\mathrm{zn}{\left({\mathrm{NH}}_3\right)}_4\right]}^{2+}4\right]$, hydrochloric acid is added to the solution. we have to increase significantly the concentration of five  ${\mathrm{zn}}^{2+}$ion in solution.

So, we have to make equilibrium shift in reverse direction this can only happen

if concentration of reactants is decreased. so, when we add some HCl,

${\mathrm{NH}}_3+\mathrm{HCl}\to {\mathrm{NH}}_4\mathrm{Cl}$ (Ammonium chloride salt)

Due to salt formation  $\left[{\mathrm{NH}}_3\right]$ will decrease and more  ${\mathrm{zn}}^{2+}$ion will start producing in solution.