12mm in a 2ft tall tank is overkill. 8mm would be fine for an experienced builder, but if this is your first "big" tank build, 10mm would be better
As above though, you will need to brace it. you'd get away with 4 cross braces on the top, but 5 would be better, with runners on the front and back edges at
both top and bottom to get the gluing area.
If you haven't built a tank a foot or two below this size before though, I'd call in a pro. Large tanks put a lot of pressure of their seams, so the margin for error is small. Mess it up and around 1000l of water finds it's way onto your floor very quickly. That's messy, expensive and inconvenient, especially when you consider most insurance companies will not cover the cost of an aquarium leaking and damaging home interiors
All the best
Rabbut