I hope you don't mind a critique or two.....
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All the weight from your tank is going to bear down on the 4 corners. The 2 x 4 on flat "foundation" that the middle posts are sitting on is going to flex under any weight, unless you've already compensated for that under the floor joists, and the shear strength from the screws on the middle braces isn't a lot. Properly supported posts under those braces would help.
Mitch