Decreasing water pressure toward top of tank walls A contractor who is building a freestanding water storage tank of 6m (length) x 2m (width) x 1.5m (depth) suggests stepping out halfway up the walls (fibreglass sat in an iron frame) by 2cm to increase structural integrity of walls (i.e. measurements of lower section of tank 5m x 2m and upper half 5.04m x 2.04m so they do not bow outward.  This makes sense in terms of putting this profile in to strengthen the fibreglass but they also state the water pressure at the top will be less in doing so.  Is this correct and what is the scientific reasoning?
 A: There is more pressure as you go deeper in the tank - so making the wall thicker as you go deeper makes sense.
The reason for the higher pressure is the weight of the liquid column above you. Imagine a thin tube (1 cm$^2$ cross sectional area) filled with water, and with a piston at the bottom. If you fill the tube to a height of 1 m, the mass of the water (at a density of 1000 kg/m$^3$) will be 100 g, and the weight about 1 N. This means there will be a force of 1 N needed to hold the piston in place.
If you now had a second hole at the bottom of the tube, but at right angles, and it had the same dimensions, then the piston would again need a force of 1 N to keep it in place, as pressure is isotropic (because water is a liquid).
So pressure is linearly related to depth (for an incompressible liquid). You need a thicker wall near the bottom of your tank to hold in the greater pressure. Of course you can make the whole thing of equal thickness - but then you are using much more material than needed near the top. Building up the lower part of the wall only makes sense.
On the other hand, the statement

the water pressure at the top will be less in doing so

does not make sense. The pressure of the water is not a function of the area (unless you say - the area is larger, I have the same amount of water, so I have a lower height). And for a circular container (yours is rectangular if I understand the description) the hoop stress (the stress on the wall) is greater (for a given wall thickness) when the area is larger - it scales with the radius of the tank. So that's backwards.
