Interesting discussion. Before I got my tank, I went through all the thoughts as you all are having too. My tank isn't on the second floor, but it wasn't in the basement either. I had an "oh sh*t" moment after I ordered the tank and figured I better do something sooner rather than when it's too late and the tank was home and filled.
Sharing my experience and what I ended up doing.
Crawlspace: Fortunately my tank would happen to be placed exactly on top of (running parallel) to the 4x8 in this picture: This was strictly a coincidence.
I was still a bit paranoid, and with the help of one of my bro-in-law added two 4x8 beams running parallel on each side of the original beam. We thought about sistering the cross beams, but felt it really wasn't necessary at this point. Yes, it was a pain in the ass getting the 8 foot beam through the ventilation opening, but it worked. I'm not as paranoid now.