  Low-temperature magnetic resonance study of the quasi-two-dimensional
antiferromagnet Cu(en)(H$_2$O)$_2$SO$_4$ (en = C$_2$H$_8$N$_2$) was performed
down to 0.45~K. This compound orders antiferromagnetically at 0.9K. The
analysis of the resonance data within the hydrodynamic approach allowed to
identify anisotropy axes and to estimate the anisotropy parameters for the
antiferromagnetic phase. Dipolar spin-spin coupling turns out to be the main
contribution to the anisotropy of the antiferromagnetic phase. The splitting of
the resonance modes and its non-monotonous dependency on the applied frequency
was observed below 0.6K in all three field orientations. Several models were
discussed to explain the origin of the nontrivial splitting and the existence
of inequivalent magnetic subsystems in Cu(en)(H$_2$O)$_2$SO$_4$ was chosen as
the most probable source.
