Density fitting is a standard technique in quantum chemistry as it helps to accelerate certain parts of a calculation, such as the computation of the electron repulsion energy, without significant loss of accuracy. This paper explores the effectiveness of this technique when it is extended to treat interactions with external electrostatic potentials, in particular those that arise from the environment in hybrid quantum chemical/molecular mechanical calculations. It is found that fitted densities are able to well reproduce the energies, forces, and properties obtained using unfitted densities, as long as a suitable operator is employed for the fitting. It is expected that this precision would be improved by the development of basis sets specifically designed to treat these types of interactions and that the use of this approximation could lead to substantial speed-ups in large hybrid potential simulations.