I ran the FEA for you. I used the outer dimensions you listed as the water volume, and assumed that you would add 1Xtotal water volume in weight of sand and 1.5 X total water volume in weight of liverock. I assumed 8.4 lbf is the weight per gallon of your water. This gave me a total supported weight of 3669 lbf. Below is a screenshot of my calculations. I used 2in x 2in hollow steel tubing, with a 1/8 in wall thickness and plain carbon steel as the working material model. The stand is 108 inches long with four evenly spaced support beams on the front and the back. The support beams are 32 in long, giving a total stand height of 36 inches. The stand is 30 inches deep. The material model assumes no defects within the material, and equal properties in all directions (homogeneous isotropic material model).
First I ran a static FEA to determine if the internal stresses will exceed the yield stress of the steel. I said the bottom of the stand (part on the ground) cannot move and applied the calculated load to the upper surface. I used the von mises stress as the failure criteria. Below is a screenshot of the results showing the internal yield stresses automatically scaled to the maximum induced stress.
Then I normalized the stress plot with the yield stress of steel. Below is a screenshot of the normalized plot.
Observation of the normalized plot, I can say with confidence that the stand will not yield due to overloading the material. My next step was to determine if the stand will buckle, which is a more critical failure mode with long tubes. I ran the buckling simulation with the same boundary conditions as the static test shown before. I only found the first buckling mode, as that is the lowest loading condition and typically the first to fail. Below is a plot of the buckling test with true displacement scale and a 45X displacement scale. Be sure to read the next part, which is my explanation of the buckling analysis.
My FEA calculate a buckling load factor of around 263. This means that for this buckling mode to take place, the load would have to 263X the load applied in the simulation. With such a high buckling load factor, I can safely say that this stand design will not buckle due to the loading condition I applied in this simulation.
In summary, a cabinet with 2x2 hollow steel tubing with 1/8 inch wall thickness tubing using 4 equally spaced vertical supports in the front and the back and a support rim at the top and the bottom should be able to hold your tank full of water, rocks, and sand.
