I think you'll find a variety of answers to that question. Since this is a build from scratch, I would suggest building it with a floating bottom panel. If you can get a bottom panel that's tempered, I would build it with the four panels resting on the bottom.
I'll just throw out some random words of caution:
That's a very long span for a DIY tank. The stand you build will need to be extremely well thought out, be on a perfectly even plane on all edges, and beincapable of warping or moving over an uneven floor. Shim the stand if needed. Glass that isn't tempered is very unforgiving.
Make sure you plan ahead and have plenty of help putting the panels in place. The weight is significant and not a one man job be any means.
Unless you're really good at cutting thick glass, have the panels cut for you and makes sure they know how you'll be using them and the need for precision.
Buy LOTS of razor blades and blue painter's tape.
"Why can't my wife see all of this stuff as an investment?"