Garf says it's OK, but they show a tank that size having euro-bracing
I agree with Brad. 11" of water should be safe.
If you're nervous, add a front-back 2 or 3" brace
http://www.garf.org/tank/buildtank.asp
Tank Information:
Material = Glass
Tank Height = 16"
Tank Width = 20"
Tank Length = 27 "
Glass Thickness = 1/4"
Approximate Gallons = 37