If you lift up the tank and look at the bottom, its raised up from the edge of the frame a bit, right?
If so you don't need anything, however, mine is a glass one and I put a thin black foam I found at Home depot under it, more for my peace of mind than anything else. Everyone told me it did not need anything.
As far as warranties on the tank go, you should be able to find the tank Manufacturer's website pretty easy, they should have instructions posted for their own tanks.
Hope that helps a bit