By "upstairs", I assume you mean 1st floor as aposed to the basement. If you really want the tank on the 1st floor, w/o question you need to distribute the weight of the four legs. To do this you can build a honey comb pedisal sandwhiched between two pieces of plywood. Trim it out to match your canopy or your existing base trim--don't leave it gheto or you might as well leave it in the basement. You also must be perpendicular to the floor joists.
The best wall will be the one adjacent to your garage--less deflection due to proximety of the concrete (interior wall--less subject to temperature changes & exposure to direct sunlight). You may even wish to double your floor joist based on their spacing--its not hard if your basement ceiling isn't finished--just sister (place the new joists next to the existing (crown up) and nail to the existing).
If you are still concerned, frame out the section in the basement under the tank--again, nothing gheto--build that large display cabinet your wife has wanted--you win she wins everyone's happy.
Happy Building,
Kurt