Some tanks are made of steel, but most are made of aluminum or polyethylene. Whatever the material, once installed, the tank shouldn't move. Interestingly, this is contrary to American Boat & Yacht Council guidelines, which allow for up to a quarter of an inch of movement. This is one of the few A.B.Y.C. guidelines with which I disagree, and I believe with good reason. Imagine that you're making an extended ocean passage and that with every wave you encounter, your fuel tank moves back and forth even slightly. This can cause it to chafe and eventually leak.