Tanks only need to be supported on the ends. I'm not an engineer, but if I was going to build a stand longer than the tank, I would have vertical supports directly under the ends of the tank. In fact the guys over at Tidal Gardens have no center brace at all. They just stack up bricks on the ends.
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If your stand has more or better support in the middle than the ends of the tank, it has a high risk of breaking the tank. Glass is stronger under compression than tension.