Is it a typical glass tank with a plastic rim? or rimless glass, or acrylic?
If it has a plastic rim on the bottom, that is all that needs to be supported. Even if you had a solid platform, the glass wouldn't flex enough to need support in the middle.
If it's acrylic, the entire bottom should be evenly supported (if you are putting it on an open center stand, get a nice chunk of plywood cut to fit, and then you can set the aquarium on that)
I'm not certain about rimless tanks. I would think that they would be fine supported by just the edges, but I'd like to hear from someone with experience on those.