I am on the basement level. The reason I cant go any bigger is this apartment I am living in was made in the late 60's and the cement pad is not only not a uniform thickness but is cracked to hell. In fact its the reason my 90g busted the first time, when the ground unthawed in the spring the rear corner sunk 1/4" and took the wood stand off level and bottom busted out.
Which is also another reason I was kinda leaning towards a 8' to spread the weight out over more area

if your thinking FO with large angels then 150g, no matter what the dimensions, isnt big enough IMHO.....gotta remember that even if the tank is 8' long the fish still need room in the other two directions....a 10" long fish will be cramped trying to turn around in an 18"w tank....

It will be 20" wide not 18", and I might decide to build a tank just like griz's and make it 22" wide as it's only 15g more

If the floor is cement you are not going to risk anything more by making it longer with the same width and height as you will be increasing the weight distribution

The owner of the building also doesn't want that big of a tank incase of a failure. But we're getting away from the point of this thread