Aesthetics fail here for me, I'd sooner have a metal frame than a central brace.
I'm planning to knock through into a garage for a "viewing room/library" for a 2.8x0.6x0.4m sized tank. Even at such a low depth, I'd need 20mm glass for approx 0.02mm deformation. 20mm does give quite a large surface area for silicone, however calculating the actual force scares me, and I'm thinking of going to about 50cm, and having a custom stainless steel cage made. A journal for this, one day.