Aquarium manufacturers sometimes use low iron glass for large tanks to cut down on the green tint thick glass can give the water. It would be very unusual to find a bow front tank with a panel that was parabolic in shape, but perhaps Oceanic or All Glass Aquarium can hook you up with their suppliers.
Some tanks come with the back pane as a mirror from the factory.
They might be able to provide parabolic mirrors on a special order basis.