ISM constraints for J1148+5251 from the FSL ratios predicted by CLOUDY for XDRs with hydrogen column densities N_H/[cm⁻²] = 10²³ (top panel) and N_H/[cm⁻²] = 10²⁴ (lower panel). The observed ratio constraints from the aperture-integrated fluxes are plotted in solid lines, and the ±1σ values in the shaded areas of the same color. The ratios absent from the plots (such as the [C II]_{158 μm} ratios) are not reproduced by the XDR grid of models.