Thank you for the reply. I did some searching around, and I think I found a YouTube video you did, is that right?

I read some of the comments and also in other places I hear people say the sheet should not be under the whole tank bottom in one like mine that has the frame around top and bottom. You replied that it should not matter, I agree and cannot understand how it would affect the tank negatively.
But then I just read in a forum, fishforums dot net, this guy who was a moderator says-
“There are two common types of tank construction.
One has a piece of trim around the bottom of the tank that raises the actual tank bottom a fraction of an inch above the stand. A pad or mat is a terrible idea for one of those tanks, it removes the small gap that the raised bottom provided and makes the tank more vulnerable to local pressure points.”
So this guy was a forum moderator, he must have some understanding greater than the average Joe, right? I can only imagine that perhaps over time as the foam compresses where the frame touches it, that eventually the bottom glass will come in contact with the foam and there will be upward pressure?