I'd start filling it and see if it bows. There isn't any danger doing this for testing. I had a 4" tank that lost it's brace in the middle of the night. The tank bowed out about 2.5" front and back and was fine for a day until I repaired it. So you could fill yours, see if it bows more than an 1/8" or so, if not, skip the brace.
If you're not comfy with that, then yes, a 6" brace would be fine.
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Brad
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