Good on you for having a go at DIY'ing a tank... and a big one too! :thup:
I agree with squirt, 12mm would be required for 700 high. It's the height that exponentially increases the glass thickness required. Drop the height down to 600 and you may get away with 10mm. As far as building it, the front, back and sides would sit on top of the bottom and the sides sit inside the front and back. Therefore for a 1500x600x700 in 12mm you would requrie the following sizes:
Bottom = 1500 x 600
Front and Back = 1500 x 688 (700 less thickness of bottom if you want a true 700 high)
Sides = 576 (width less 24mm for front and back glass) x 688 (as above)
Of course on top of this you'll need bracing and lids.
Good luck and keep us posted! :thup: