How froze you ask? Well just so happens I had a couple huge chunks of ice from Titus' water bucket laying around --- so I threw them into the tank. the first one barely cracked the surface, but the second one put a little hole in the ice. I would guesstimate tthe thickness to be about 1 inches thick.
Here's Titus a bit concerned as to why I'm so violently throwing his neat new chew toys (the ice) onto the tank?