The weight of the water raised in capillary tube is balanced by the force excerted due to decrease in pressure inside the water column due to the curvature at the water surface.
The decrease in pressure = $\frac{2\mathrm{a}}{\mathrm{r}}$
from balancing pressure, we get.

$\frac{2\mathrm{a}}{\mathrm{r}}=\mathrm{h\rho g}$

∴ h ∝ a

since, the surface tension (a) decreases as soap mixed, the height upto with water rises decreases.