According to the glass thickness calculator here: http://www.fnzas.org.nz/glassthickness.0.html
IIRC the spreadsheet assumes a well braced tank, so yours has a safety factor of 1.5, recommended is 3.8, which would require 10mm glass.
Basically meaning your tank, full to the top with average glass strengths will probably break if you put 50% more force on the glass. The safety factor goes to 2.0 if you only fill it to 50mm from the top.
3.8 is recommended to account for things like little earthquakes, people bumping into the tank, rocks shifting, fish hitting the glass etc.