Also when building the aquarium is it the height of the tank that measures how thick the glass has to be ?? say i was making an aquarium 79"x24"x18tall on grafs it says i only need 1/4" glass but if i alter the heigth it says i would need 3/8 to 1/2"
It's height and length. You might get away with 1/4" if you have a few braces along that length. I would go 3/8" just to be sure and run at least one center brace across the top, 2 if it doesn't impact your design.