The best way is to look at all of the possible points of failure:
1. buckling of the legs in three ways:
b. bending in the x direction
c. bending in the y direct ion
2. Bending of the cross beam to the poin of breaking
From experience you can rule out number one after seeing your construction method. Therefor number two is the key factor. (If this were a critical calculation, all posibilities would be calculated to rule them out)
You can use this calculator to determine the deflection. But remember, if you are calculating the deflection on a 2x4 you can only count the weight that is on that specific 2x4. In order to do this, calculate the contact area between your tank and the stand. From this calculate the pressure exerted on the stand xxx lbs per square inch.
From this then use the worst case scenario, which is an unsupported 2x4 with the 4" side against the tank. Thus the contact area is 3.5" x the tank length.
Use these values in the calculator:
W=tank weight/7 (# of 2x4's) (i'll use 1540# for the whole thing)
X=length of tank/2 (the worst bending will be in the middle)
l= length of the tank
z=.75 (half of the thickness of a 2x4)
based on this, the beam will bend 5/8" at the middle.....too much
you want no bending. So we try 2x4's on the short side
I = 5.35
And then you maximum deflection is .05" Very acceptable...
This is also using the modulus of elasticity for a lower grade of lumber than what you'll find. Also, this model assumes that you only have the tank supported at the four corners.
I increased the load to 5000#, and the maximum bending only increased to .15". Still acceptable.
Let me know if you want more info.