120 gal is approximately 1000lbs or 5 large people. Add another 500lbs for rock, glass, sump, etc. Get some pizza and invite the appropriate number of friends over and test
Next part is key: you need to all jump up/down at the same time in the spot you want the tank to be. If you fall through the floor or the building collapses, then you should not put the tank there!
Seriously, if you place the tank against a load bearing or external wall, and the length of the tank is cross-wise to the joists under it (in other words, you span 4+ joists, instead of 2)... then you should be fine. As always, IANASE (I am not a structural engineer), so get a professional to check it out.