Safety factor for 6mm glass on a plywood tank that size would be 1.08.
So I'd never consider using 6 mm glass. I wouldn't trust glass that thin for any DIY water vessel project over 4 foot long or 50 gallons. It's just asking for trouble. A 6 mm x 67in might break before you install it since it could easily fail just from it's own weight, if you handle it wrong. Need to go thicker.
8 mm / 1.9 safety factor wouldn't be recommended either, but could work depending on the tank design. If it were an all glass tank, the glass on glass siliconed joints would brace themselves, making the seals much less likely to fail. The problem with plywood tanks, is that usually the window is just siliconed into place on top of the plywood and the window isn't braced by anything. This decreases the safety factor. Most glass thickness charts or calculators are for glass only tanks, not plywood tanks. So in order to use a window of this thickness, the window will need to be supported and braced on all 4 edges, not just siliconed into place. BTW, I have written a glass thickness calculator specifically for plywood tanks. I won't publish it though, because some people don't understand that the larger the tank, the higher the safety factor is needed.
12 mm / 4.28 safety factor would be where I'd be comfortable for a plywood tank.