  Recently neutral and charged black-hole solutions were found for static
perfect fluid with the equation of state $p(r)=-\rho(r)/3$, for fluid only as
well as for fluid in the presence of electric field. In those works, the
stability of the black holes were studied in an analytic manner, which
concluded that the black holes are unconditionally unstable. In this work, we
focus particularly on the {\it numerical} study of the instability. For the
black-hole solutions as well as the static solutions without horizons, we solve
the perturbation equations numerically and find the unstable mode functions.
