roughly, 1 gallon = 10 lbs..
Also unless you know for sure, its pretty safe to bet that the builders went with "minimum" mandatory req's, which in canada is 16" between floor supports..
Your 180g tank with stand and rocks and filters will weigh well over 2000 lbs if fully filled..
You can put wood under the stand to spread out the weight over more floor beams, that is what I do with my 180g on third floor of wooden building..
However I only fill my tank half - 2/3rds full..