union carpenter here,been building homes for 20 years. if your not sure what your 2nd floor deck was framed with,2x8...2x10...2x12. another bit of helful info is the span of your floorjoist...longer span =more bounce. i would definately stay near exterior wall,not a patition wall but exterior wall.I would also recomend useing a stand that will distribute weight evenly as opposed to something with legs that could poke through plywood or OSB if leg isnt sitting above floor joist..hope that helps, good luck.
Oh btw make sure stand is level before filling tank.its not unheard of for older homes to to be out of level and plumb