I'm also hearing in what is req'd to have such a tank.  From some, I have heard that one must have a retaining wall around the tank that will hold the entire quantity if it spilled; and then from others, I've heard you don't need any of that.

 A neighbor of mine recently had a 5-6k gal. brought out.  It was on skids (new tank), and he just set it down on a level spot & called it good. He put in a station-style pump, and plumbed it into the bottom of the tank.

 Why do some say that the retaining wall & all is req'd, and other times you hear of nothing but the tank??  I would be interested in knowing what is actually required.