Nikki - wrote,
If I take my 55gl down and put a 120 gl in its place, should it be ok, is
there anything I should look for?

Look to see which way the floor joist are running where you plan to put
the tank. It's always better to put a large tank against an outside or
center wall where there is eather a foundation or beam supporting the
ends of the floor joist the tank will be sitting across.
.............Frank