It depends on the age of the home (what codes were in effect when it was built). Typically a live load of 40 psf is the minimum design spec and most likely don't exceed that due to weight and cost.
As mentioned above, water is 8.33 pounds/gallon - rock, sand, and glass are heavier. So using 10 pounds/gallon will give you a safer estimate.
150 gallons would be roughly 1,500 pounds spread over ~15 square feet or ~100psf
The issue here (as mentioned above) is not typically going to be complete floor failure, but instead significant deflection and bouncing.
It depends on the foundation and climate (freeze thaw, etc.)
The load could be supported from underneath - (as mentioned above) this depends on the foundation. Is it on slab, on runners, on piers, etc.
on where in the home the tank is going.
The center of the room, against the wall, over the main beam, etc.
In general, bigger tanks are going to shake a good bit due to the lack of rigid structure.
As you have already surmised, it would be best to see the home first and get some details and then make a decision according to the data that you gather.