20 Gal on home-made stand: small GAP in front middle
I just set up a 20gal tank on a home-made stand, well, actually it is an old kitchen cabinet with the counter top removed. I thought it was flat, but it turns out the front sags somewhat, this is from age and use, and I don't expect any more sag to occur... however, I knew there was about 1/2 inch sag in front, so I put the tank on a 3/4 inch foam (high density) slab... figured it would sink into the foam on the outside edges and even out... well it's been up for 24 hours full, and the foam seems to not be compressing at all... there is still a gap in front along the bottom of the frame, in the middle, about 1/16 inch, but all the way along the front getting less toward the sides. I would say the sides for about 3 inches touch the foam, but then the gap starts... as I said, worst in the middle...
Why isn't the foam compressing to take the load evenly?
What should I do now? Some have said don't worry about it bc it is only 20 gal.
I don't want to stress the fish out out and drain the tank... should I shim the middle or will the foam eventually compress? What should I shim with and how tightly? Can I shim without draining the tank?