  Coupling dark energy and dark matter through an effective fluid description
is a very common procedure in cosmology, however it always remains in comoving
coordinates in the special FLRW space. We construct a consistent, general, and
covariant formulation, where the interaction is a natural implication of the
imperfectness of the fluids. This imperfectness makes difficult the final step
towards a robust formulation of interacting fluids, namely the construction of
a Lagrangian whose variation would give rise to the interacting equations.
Nevertheless, we present a formal solution to this problem for a single fluid,
through the introduction of an effective metric.
