I just measured and the height is identical on the dot.Levelling mat as far as I know is to compensate for variances with stand not being level. I don't necessarily think that it is for fixing gaps. If you measure the height of the stand from the floor or bottom of the stand from the corner that doesn't have the gap and on the corner with the gap, is the height identical?? It almost seems as if there's some warping....I hope not! Filling water into a tank that isn't fully supported at the bottom glass is like filling a tank that's sticking out from the edge of the stand....weight bearing down on the glass stressing the silicone seal.