The conditions of existence of extra mass flux in single component
dissipative non-relativistic fluids are clarified. By considering Galilean
invariance we show that if total mass flux is equal to total momentum density,
then mass, momentum, angular momentum and booster (center-of-mass) are
conserved. However, these conservation laws may be fulfilled also by other
means. We show an example of weakly non-local hydrodynamics where the
conservation laws are satisfied as well although the total mass flux is
different from momentum density
