Detectability with PACS on Herschel of some diagnostic far-IR fine-structure lines discussed in this paper, which can be used to measure the gas metallicity, ionization parameter, and density in galaxies, as a function of redshift. The left hand scale gives minimum L_FIR to achieve a 5σ detection of each line with an exposure time of 3 hours. The right hand axis shows the minimum star-formation rate for the line detectability corresponding to the minimum L_FIR based on the relation given in Kennicutt (1999). Green, red, blue, and magenta lines denote [Nii]121.7, [Niii]57.21, [Oiii]51.80, and [Oiii]88.33, respectively.