The foam will compress with time, but, if the gap is too much then you need to level it, when it´s only a few milimiters there´s not much problem but if it´s more you have to level it. 20 Gal may not seem as much but got any idea how much a 20 gal tank weights when full ? the answer is around 200+ pounds and that´s a lot of weight, weight that has to be evenly distributed over the standing surface because glass is anything but solid, glass bends and deforms under pressure, pressure caused by the water itself and in a tank the pressure points are in the middle of the long glasses that form the tank; specially in the bottom glass with time the silicone adhesive will break because the pressure pushes the bottom glass this pulls the adhesive and the adhesive breaks. I know it because I used to build tanks when I had the aquarium store and before installing the tank I had to level the surface with styrofoam so the tank would stand leveled at the bottom. Shim the middle. How thight ? until you have filled the gap.

May be a good idea to use a piece of 3/4 MDF or plywood over the top of the stand. Shim the gaps as raul said.

Yes put that flat piece of plywood under the tank and the styrofoam on top of the plywood then there should not be any gaps.