[Francis-134]

While I'm not an expert on plastics, I have found that the only advantage to plexi over Lexan is cost (a cursory inspection of Wikipedia confirms this). Plexi tends to cost much less than Lexan.

For example, the large tank's viewing screen at the Atlanta Aquarium is made of acrylic not Lexan because it's never going to come under any significant shock loads and will never risk fractuing. If it were made of Lexan, it would probably cost MUCH more money.