Old 2x4 are stronger, they are actually 2 inches by 4 inches. We have 3.5 inches by 1.5inches (roughly). Apparently it costs too much to make actual 2x4s.They old ones are bigger, thus support more weight.
Also, i would say that a 150, complete with sand, rocks, filters, and a stand is too heavy for a floor that isn't supported with concrete or extra beams.
A floor must be able to hold 40 pounds per square foot in order to be safe for a single family residence. However, it isn't quite set in stone. I can have a 75 gallon tank, which weighs around 750 pounds, on the 6 square feet of floor that it takes up, and it will be ok, even though its about 125 pounds per square foot instead of 40. However, I've every single square foot of my floor held 125 pounds, it would shear the supports and there would be a big mess. Also, if a tank is going near a wall, then it is safer than if it was going in the middle of the room. You want it perpendicular, NOT parallel, to the floor supports. This all applies to a floor that isn't supported directly by the foundation or extra supports, such as walls underneath or added 2x4s. A 150g is likely to weigh about 1600 pounds when complete, give or take. It isn't that much more per square foot that a 75, however i personally wouldn't recommend it.
Also, you have to factor in how far apart each beam underneath the tank is, how old they are, if there is any water damage, etc.
If you are putting this tank down where it can go perpendicular to at least 5-6 2x12 joists, and is against a wall, and the house isn't brand new or too old, then it might work. It also shouldn't be crossing the same supports as any other heavy structures, like furniture or a bed, because that puts a lot of strain on the 2x12.
Ive done a lot of research on this, and i can tell you that it would be much safer to put this on concrete...
Beams/supports/joists are all the same thing in this, sorry i didn't use one name consistently.
Sent from my bed, the only space available to me that isn't smothered in dirty laundry or aquariums.