Why can't I do this with my stand?

[Tom R]

Depending on the tank size.

A 10G tank weighs 100 to a 130 Lb.

A 180G tank weighs 1800 to 2200 Lb.

You are wanting to distribute the weight over as big an area as possible and as many floor joists as possible.

Those legs are about 1 1/2 sq inches. That would spread the weight over approx 6 square inches.

For the 10G tank that would be approx 22 Lb. per square inch.

For the 180G tank that would be approx 367 Lb per square inch.

For larger tanks 2 x 4 or 2 x 6 construction shimmed for level is by far the best.

Tom R