I don't know if it helps, but when I bought my 90-gallon, the guy at the aquarium store said that was the largest aquarium size he would recommend for any regular floor (non-concrete slab) and that any larger tank would require a concrete slab floor to properly support it. Also, he said that chances are I would only fill it up 2/3 of the way ("60 gallons"), so he was confident it would be fine since I was never going to fill it up all the way. FYI - I live in a condo that was originally built in the 70's with a basement, and my tank resides on the main floor of the house.
If your 55 gallon isn't going to be completely filled up (fill up only 1/2 way so that you have room for the basking area at the top), then technically you only have the equivalent of maybe 25-30 gallon tank which should be fine on the 2nd floor.
