With only 20" from seam to seam the side panel is less likely to bow under pressure and crack, as opposed to the longer span of the front or back panels. In any case that is quite odd. My concern would lie in that the front and back have a much thinner butt seam where it attaches to the sides. I've got a 4'Lx4'Hxx18"W tank, but the sides are made of 3/4" glass just like the front back and bottom. Previous owner had it in service???