My 3ft (180L) tank is only 5mm-6mm glass. I think Juwel also manufacture the 240L (4ft) which is also the same glass thickness if memory serves me right. If that glass thickness was a safety concern, I don't think Juwel would be pumping out (excuse the pun) the amount of tanks that they do. Just take care to position and seat the tank correctly and don't drop anything heavy onto the bottom pane.