frag tank build

[donkey77]

ok got a sweet deal on a frag tank its 5 feet by 5 feet by 12 in high, 187g the guy i got it from had plywood bottom with pond liner in it, well it didn't fit downstairs so i took it apart, the glass is 3/4 " thick i'am going to cut 2 down to 3 feet, so it would be 5 feet by 3 feet, aim going to get a glass bottom, do i need 3/4'' for the bottom or could i go 1/2''.