Glass thickness for plywood tank
It looks like the glass needs to be strong enough for 72 inches depth.  But besides the pressure there is the span.

A standard aquarium glass thickness calculator only goes to a depth of 36 inches, but using the 5 foot width with a safety factor of 2, that calls for 16 mm glass, and 22 mm for a safety factor of 3.

Using a more general calculator and making all sides glass for a 256 x 366 x 183 cm aquarium using a safety factor of 3 gives 55 mm glass (which is 5.3 tonnes of glass) and an overall weight when filled of nearly 22,000 kg (24 U.S. ton).

If people will be near this structure, of course you want tempered glass.