Okay, just to clarify the comments I will make, the floor above the basement is the first floor, not the second.
The first floor will have many strong areas (for example, the kitchen will have extra support for the appliances, the bathrooms might have extra support for tubs.) And because the first floor has to be under the second floor (in a 2 or 3 story house), it will have more strength as well.
None of that will directly answer your question however. You may be able to go to the basement and examine the floor supports: are they 2x6, 2x8, 2x12? how far apart are they spaced? how long are the beams? is there an I-beam? what size and how many additional columns do you have? what type of fasteners were used? in what condition is the wood and the fasteners? where will the tank be placed?
Most people simply place the tank and don't worry too much about it. One hundred gallons would probably not worry me as long as I could see the floor structure and it looked like it was built properly.
If the floor is exposed, a structural engineer familiar with residential homes could probably give you a far superior answer to anyone else speculating on a forum. The problem is that it's possible for example that all the fasteners may not have been used, that the contractors built the floor improperly or that some of the wood beams have wood rot. That requires a visual examination.
But if you are okay skipping that, then abide by a few common rules: placed the tank perpendicular across 2 beams, and against a load bearing wall. There are more things one can do, but those are the easiest.
