We report on the systematical investigation of Fano resonances in electrically biased semiconductor superlattices. In such quantum structures, Coulomb inter action couples discrete excitonic states of the Wannier-Stark ladder to continua of lower-lying subbands. In absorption these transitions show the typical asymmetric line shape of a Fano resonance. The unique feature of these Fano resonances is that they allow to continuously tune the key parameter - the coupling strength Gamma between the discrete state and the degenerate continuum - by varying the bias voltage. Using this feature, we directly show that the Fano coupling leads to a fast polarization decay. We also investigate the dependence of the Fano parameters on the structure of the superlattice and compare to an extensive theoretical model of the resonances.