  We investigate the difference between holographic dark energy, Chaplygin gas,
and tachyon model with constant potential. For this purpose, we examine their
squared speeds of sound which are evaluated to zeroth order in perturbation
theory and hence depends only on time. We find that the squared speed for
holographic dark energy is always negative when choosing the future event
horizon as the IR cutoff, while those for Chaplygin gas and tachyon are
non-negative. This means that the perfect fluid for holographic dark energy is
classically unstable. Hence the holographic interpretation for Chaplygin gas
and tachyon is problematic.
