Tetraman

Hello there, I have just been given a tank 200cm long, 60cm high, 40cm wide and is made fo 8mm plate glass. I understand for a tank this size 8mm is not safe. However if I were to fill it upto about 35cm or 40cm then will it be safe? Is it the same as having a tank 35cm or 40cm high? If I fixed a couple of braces on top, how high can it be filled safely? Thank you.