Are the 2x6 rails between the vertical 2x6s supporting the tanks corners?
The stand is supported by 5 2x6 on all 4 corners 4 2x6 directly under the tank transferring load on 4 corners to 8 2x6 and sitting on 1/2” ply. Also 2x6 Rails. I didn’t need the center uprights if I had a 8 ft tank but I put them to build a stand within a Stand. My sag is zero using digital calipers and a full tank with a string tied and measured a known distance from the floor before filling.