  Context. We present our findings on 18 formerly known ZZ Ceti stars observed
by the TESS space telescope in 120s cadence mode during the survey observation
of the southern ecliptic hemisphere.
  Aims. We focus on the frequency analysis of the space-based observations,
comparing the results with the findings of the previous ground-based
measurements. The frequencies detected by the TESS observations can serve as
inputs for future asteroseismic analyses.
  Methods. We performed standard pre-whitening of the data sets to derive the
possible pulsation frequencies of the different targets. In some cases, we
fitted Lorentzians to the frequency groups that emerged as the results of
short-term amplitude/phase variations that occurred during the TESS
observations.
  Results. We detected more than 40 pulsation frequencies in seven ZZ Ceti
stars observed in the 120s cadence by TESS, with better than 0.1 microHz
precision. We found that HE 0532-5605 may be a new outbursting ZZ Ceti. Ten
targets do not show any significant pulsation frequencies in their Fourier
transforms, due to a combination of their intrinsic faintness and/or crowding
on the large TESS pixels. We also detected possible amplitude/phase variations
during the TESS observations in some cases. Such behaviour in these targets was
not previously identified from ground-based observations.
