If your buying the tank I wouldn't worry about it, acrylic tanks use Euro Bracing at the top, the tank won't bow with this set up.
I had a glass tank that big and know that these things are all kinds of braced for a strong structure. If your making one, then I'd go with 3/4", and possibly a big center brace.
I had a glass tank that big and know that these things are all kinds of braced for a strong structure. If your making one, then I'd go with 3/4", and possibly a big center brace.