Do you mean the Nuvo Fusion 20 gallon or the SR-120? I don't believe there's a 20 size in the SR series.
The IM Ghost Skimmer sometimes works and sometimes doesn't. Mine will make a little tea colored skimmate occasionally but most of the time it just makes noise. It is definitely the best fitting skimmer for the tank since it was designed specifically for IM tanks, but even then there are other skimmers out there that, with a little customization, will fit in the back and will outperform the Ghost skimmer.