Do you live in an apartment complex? We do, and we called up maintainace and asked them if it can handle our 90 gal and they said no problem. Actually where we put it is a great place (according to the complex) b/c there's also a cross support to our patio/deck and the weight of the aquarium reinforces the deck! If you have your own house you're going to have to get the blue prints or something that shows you where all the major supports are. Or have someone experianced come and check it out. We actually had to sign a waiver saying that if it leaks we're liable.
It depends on the joists size and space between them. You want it going across as many joist as possible. I have a 150 along 3 joist with no problem. I did notice some slight bowing after the first few months and re-supported those with 2 x 4's.
