I have my 120 with 60 gallon sump in my Doublewide, My joists span 13 feet across on the side with my tank . I dont have it on an out side wall and have no probs at all. As a matter of fact the manufacture of my home told me to take the width of the tank x the length of the joist x 50 and thats what my floor can hold . But mind that is furniture and people included
For me that was
2600lbs. So my 1500lb 120 is safe and sucure.