the question of how big a tank can you put upstiars comes up all the time.
there will undoubtedly be people popping up soon enough saying 'oh i have x gallons upstairs and that's fine'. however you must be careful not to take this advice.
I work in an international engineering consultancy, my other half is a builder, so i know a little about the construction industry and that side of things. the simple answer is every building is constructed differently, every building will have been specified with a particular load that can be carried at each point of the structure. Now if you've a brand new house then fine, you could possibly approach the buildiners and get an answer to this question, however if your house is older then firstly you'll probably never track down who built it, and secondly it wouldn't be safe to do so as there will have been all sorts of strains on the house over time. things like subsidance or joists rotting can affect the structure so it may not be as secure as it once was.
so i'm sorry that this isn't the answer you're looking for, but it's better than pushing it and you're house falling in!
There is really no set guideline we can give you for what is safe upstairs, yes pretty much every house can probably hold a 30gal upstairs but if you are at all in doubt then you should contact a structural engineer before you put a tank in place.
