[en] It is shown how the Hamiltonian description of an ideal fluid can be used for a two-fluid plasma. Translation to wave variables leads to a simple rule to obtain coupled-mode equations if the energy is known as a function of wave variables. In this context the intimate connexion between a particular dynamical model, specified by the interaction Hamiltonian, and the associated invariants of motion is discussed. By this method an improved version of Zakharov's equations for Langmuir and ion sound waves with the correct invariants of motion is derived. As a result of these corrections a stationary power law k^-2 for the plasmon energy is found. (author)