Most aquariums are made using glass or acrylic (plexiglass) in various thicknesses, depending on how big the tank will be and also whether the aquarium will be “built-in” or free standing.
For very large built-ins, maybe just the front “viewing” panel will be glass or acrylic and the other five sides can be wood, stone, metal, or some other structural material. For tanks holding many hundreds of gallons, the glass or plexi has to be thick enough to handle the pressure that the water weight exerts.
The best thickness option is one inch. Whether in glass or plastic, 1″ thick is more than strong enough. There are pros and cons for both options. 1″ thick plexiglass is very strong and only half the weight of glass, but it scratches easily and turns “yellowish” over time, especially if exposed to ultra-violet light.
1″ thick glass is much harder than plexi and therefore is much more resistant to scratches and will never “yellow”, no matter how old it is. However, glass is heavier than plexi and one inch thick glass gets heavy in a hurry (13.13 lbs. per square foot)! Standard 1″ thick glass (25mm, actually) which is foreign sourced can take on a very pronounced aqua green hue and thus obscure, to a degree, the view.
There is a much rarer version of one inch glass available… a “low-iron” glass called Starphire® by PPG Industries. Made in America, it has far less green than standard glass… closer to crystal in clarity. The thickest plate glass available, it can be difficult to find. One reliable source is American Glass Top Company in the Chicagoland area. They stock 1″ thick Starphire® glass, fabricate to the size you need and ship nationwide. They can be found at www.americanglasstop.com or at 1-800-793-1011.