Also you don't need to add the sump volume into your consideration since your sump is a giant filter, whereas your display is the one that's polluting the system. Unless you are feeding your sump (such as keeping fish in it), then sump volume is not an issue.
ie. If I have a 100g display and 75g sump, I don't need to buy a skimmer rated to easily skim 150-175 total volume, just one that will handle 100g display volume & bioload.
Your present skimmer should handle 120g display easily.
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