There is a material made for body/vehicle/aircraft armor applications that is similar to metal and can stop a .50 Cal Armor Piercing round within 1.5 inches. That is incredible to be able to stop that much kinetic force in such a small area. I don't know if it would be useful in aquarium applications due to the different stress points (constant pressure vs a single devastating impact) nor how much it would cost, or even if permission could be gotten for use in civilian applications, but it might be something to look into.
www.tssbulletproof.com/optically-clear-aluminun-provides-bulletproof-protection
www.surmet.com also has ALON (cooyright) armor that is very similar.