  Quark-gluon decays of heavy paraquarkonia $^1 S_0(\bar Q Q) \to \bar q q g$
are investigated with account of the masses of final quarks. The decay widths
and the energy distributions of the final quarks and gluons are calculated in
dependence on the relative quark masses. The strong collinear enhancement of
the gluon energy distribution at the end of the spectrum is shown to take place
in all such decays of $\eta_c$ and $\eta_b$ mesons except the decay $\eta_b \to
\bar c c g$. The total decay width is shown to have an essential dependence on
the final quark masses. The corresponding branching ratios of $\eta_c$ and
$\eta_b$ mesons are numerically estimated with a good agreement of $Br(\eta_c
\to \bar s s g)$ with experimental data on $\eta_c$ decays.
