The 2x4's that run the 120" length have a rating of .1095 inches of deflection with 16000PSI load at the mid point.
The longest run we have of the 2x4 is the 48" on the ends of the tank. Given the entire tank is going to weigh less than 16000 lbs spread across the ENTIRE surface area of the stand (whether you view that as the 120x48 top deck , or the 96x36 base supporting the top deck) once you spread the weight across all that surface area the actual downward force of the tank is pretty minimal.
The tank weighs 1000lbs dry - Add 600lbs of rock - add 5000 gals of water - and we have 6600lbs. Divide the 6600lbs over the 96x36 base which is roughly 3400 square inches and the actual weight transfer in any particular section of the stand is pretty minimal.
This is an all steel stand going on a solid concrete foundation.
I think we are going to be okay.
Dave B