  We report the crystal growth by chemical vapor transport together with a
thorough structural and magnetic characterization of the quasi-2D magnets
(Fe$_{1-x}$Ni$_x$)$_2$P$_2$S$_6$ with $x_\textrm{Ni} = \textrm{0, 0.3, 0.5,
0.7, 0.9 & 1}$. As-grown crystals exhibit a layered morphology with weak van
der Waals interactions between layers parallel to the crystallographic $ab$
plane of the monoclinic structure in the space group $C2/m$ (No. 12).
Magnetization measurements reveal an antiferromagnetic ground state for all
grown crystals. In the ordered state, the magnetization along different
crystallographic directions is in agreement with an Ising anisotropy for $0
\leq x \leq 0.9$ and only for Ni$_2$P$_2$S$_6$ a different behavior is observed
which is line with an anisotropic Heisenberg or XXZ model description. This
striking abrupt change of anisotropy at a critical Ni concentration is coupled
to a change in the width of the maximum in the magnetization around
$T_\textrm{max}$. Furthermore, all intermediate compounds $0 < x \leq 0.9$
exhibit an anisotropic magnetization already in the paramagnetic state similar
to the parent compound Fe$_2$P$_2$S$_6$.
