I'm not a structural engineer, but I am a builder/developer of residential properties. What you're planning on doing makes me nervous. Do you own or rent? If you rent, I wouldn't do it. If you own, there are several ways to beef up floor joists, none of which are easy without cutting the first floor ceiling. If you must do it, then do it so that it spans the most joists possible, and do it on the stairwell wall or exterior wall, because even without looking at the prints, I can tell you that these are bearing walls. Also, if your home is old, you may have a subfloor made of 3/4" slat or tongue and groove planks. These are no good anymore. You can beef up your floor by going to Home Depot and getting a sheet of 3/4" advantech. No other plywood will do, as it will never match the ridigity of advantech. You can screw this down over hardwood, but if its carpet I would trace the tank stand and cut out the carpet, and then cut the advantech to fill the carpet hole. If you save the carpet peice you can seam it back in before you move out. The advantech will tie the floor joists together much more effectively and distribute weight more evenly. I would still be nervous, but taking these steps would allow me to sleep at night. Also it should go without saying that the stand needs to be "dead nuts level" and bolted to at least 2 studs. This does nothing to distribute the weight to the wall, but it keeps the load static since it doesn't rock when you walk by it. My .02
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