Three primary considerations for the slab are:
--Strength of the subgrade/subbase
--Strength of the concrete
--Magnitude and type of loading

Since your loading is uniform and the slab is ground supported, there isn't much need for reinforcement other than to control shrinkage cracks.  Assuming your liquid level doesn't exceed about 20 feet, you will have a uniform load of about 1250 psf (assuming Sp. Gr. = 1.0 (water))At or below this value, it isn't necessary to consider slab bending assuming your modulus of subgrade reaction, K, is greater than about 150 pci.   

I would suggest a minimum concrete strength of 4000 psi, using as large an aggregate as practicable.  Reinforcement would likely consist of at least one mat of 6x6-W4.0 x W4.0, placed at the center of the slab.  The slab thickness would likely be in the 6-inch range, but you can check that analytically through any of the pavement slab design procedures (contact pressure = 1250 psf or whatever it happens to be).  A good reference for this is "Slab Thickness Design for Industrial Concrete Floors" by Robert G. Packard, Portland Cement Association, Skokie, Il.

This is a variation on a pavement thickness design problem.

Ron, Everything you said makes sense to me.  However, I have several sets of structural drawings from different firms and all have slab thicknesses of 12" up 24".  And they are reinforced using Fig 2.5 from ACI 350.  What are they doing?  Could it have something to do with the moment transfer from the wall to the slab?

There are several considerations that could lead to slab thicknesses of that magnitude.  If the shear from the wall load is relatively high (not likely, but could be), then you would have to consider that.  It wouldn't be necessary to maintain that thickness over the entire slab though.  Secondly, if the tank has a h/D ratio that would present stability problems under wind load, you would need more slab mass to resist overturning.  Finally, the soil conditions and total load might indicate essentially a mat foundation for the tank, integral to the floor.  All of these should be considered, though based solely on liquid level, the slab thickness would not be great.

I'm not an expert at all on the design of foundations.  However, I have designed many storage tanks to API 650 code.  For the smaller tanks we normally deal with, which range from 12' to 20' in diamter and 2:1 height:diameter ratio, there are 2 loads which normally govern.  The first is the seismic moment, which is normally much larger than wind loading.  The other is design for internal pressure.  As a flat bottomed tank has essentially zero stiffness in its bottom plate, all resistance to bending must come from the foundation.  The internal pressure tends to lift the shell, which must be resisted by the anchor bolts.  So essentially, you have an increased uniform load over the tank bottom, with numerous uplifting point loads along the edge of the tank.  If the foundation is thick enough, these numerous point loads can probably be considered as a uniformly distributed opposing edge load.  This causes a distributed bending moment all around the tank edge.

I agree with mann, I have never come accross a floor slab of a covered watertank with minimum reinforcment, nor have I designed a floor slab of a covered watertank in that manner. There is always a moment transfer between wall and floor, unless the tank is cylindrical, then the moments are relatively minimal. In addition, if the tank is covered an upward soil pressure on the floor slab results causing moments in that slab. The amount of these moments depend on whether the soil pressure distribution beneath the floor slab is assumed uniform or not. However, moments in floor slabs exist.

If you are going to use reinforcing in your slab for crack control, please use rebar instead of welded wire fabric. Rebar costs more but it will probably remain in the location where you want it  when the concrete is poured, whereas, welded wire fabric will probably be in the dirt!  #4s at 12 inches on center is a good rule of thumb.  Good luck!

i guess slab thickness should not be less than 12" because of the stability of the structure  (i mean in the mid span and u can increase it in the corners because of the  transfered moment from the walls..anyway u 've got to check stresses after  each assumption

I would like to add to what Mann and Hasanh have said.The base slab thickness of a water tank depends on

1.Moment at the wall and base junction. Which is more if wall is considered as cantilever, and appropriately less depending on top and side end conditions of walls of rectangular/square tanks.Much less if the tank is circular because of hoop pressure.

2.If the tank base is below the ground level and the ground water table is high, there will be an upword presure on the base slab when the tank is empty. The slab has to resist this moment and that governs the steel many times. In this case the slab bends like  any suspended slab between the walls.

3. Usually a minimum thickness of 6" to 8" is specified for water tightness irrespective of moments with minimum steel.

I am lookimg for design data to design a square concrete tank, Rebar re-inforced.
Crteria: Capacity 10,000 US gals, format square. wall height8' To Be cast in place.
Thanks for any design guide info. This is for a volenteer project(Romanian orphanage)

PCA has a publication you need to use, entitled, appropriately enough, "Rectangular Concrete Tanks".  I think the publication number is IS003.03D.