100gal on mid level

[toxic111]

it depends on how old the house is. By my quick calc's you are looking at putting 90psf on a floor that is usually designed for around 40-50psf.

Now, if your span is less than 10' and you are fairly close to supports (beams) you might be good.

An engineer would be your best bet to confirm what you can do, it should only be a couple bills to have one look into it.

BTW, you might never have a problem, I have seen waterbeds over 2x8 joists.

A larger tank might work, you just have to see what the final loading will be. If you can get things to under 75psf I wouldn't worry about it.