If the soil is compressed, you don't need to do anything to prepare it. If your pond walls will be block, then you should pour a concrete footer for them, but the pond floor is fine without concrete.

What kind of glass do you have? Is it tempered? Are you sure it can handle the pressure from that much water?

Hi
The glass is from entrance doors from a remodeled building. Got them free which is what inspired me lol.
I feel the glass will be more than strong enough since it
can't be more than 24 inches deep. in aquariums they use half or 5/8. Was going to use 3/4 marine plywood for the
rest of the box with a built in stand to raise it above the ground. Was thinking between 24 to 36 inches wide.
Would like to put it directly on the ground but concerned with "settling' . A footing would waste so much space underneath.
I setup a standard 150 gallon aquarium in the shadehouse using crushed lava rock as a base. Has worked
for two years now and seems to be staying level. Only problem has been that the rock sticks to your shoes..
Of course this setup will be at least twice as much weight!!. Thanks for the help gary

A concrete slab would be the strongest and most level. But perhaps you could also consider a brick or stone "patio?" This way if you ever change your mind you're not stuck with a slab. That will be one very heavy beastie.