I live in a manufactured home myself and I have a 150 also, here is what I have found to be the case.
#1 I had a 55 that was going with the joist and the weight did begin to sink the floor because it was between to joist.
#2 I have a 75 that goes across the joist no problems there
#3 Where I want to place the 150 is with the joist and I will only get one to support the tank. So after talking to the manufacturer and a home builder what they recommend is cross supports. This can be done 3 ways.
A: Place another one or 2 2x12 (size of my joist) between the other joist creating one or 2 more joist.
B:Using 4x4 create t braces spaced 12" between the joist. So basically take a 4x4 attach it to the joist against the floor and then create a down leg to help transfer the weight to the slab underneath.
C: Create a ladder underneath by tying 3 joist together every 12 inches and then add 2 down supports under the joist themselves.
Both said if I did not reinforce the floor that I would have 1 to 3 years before it would start to sag. I will probably do the 4x4 brace system since it is the easiest but that will be a summer project.
Hope that all make since.:huh: Oh by the way my house is only 3 years old so on an even older home I would imagine bracing the floor is an even higher priority than on a newer home.
You have to figure the weight of the tank if it is an older one is 200+ lbs + 10 pounds per gallon + decorations say 30lbs plus the weight of the stand you are probably pushing 2000+ pounds