Quote by Michael C
The formula in the previous posts is correct if we are using the usual definition of density, which is mass per unit volume. It will give correct results if we are using SI units, or any other absolute system of units.
Your formula is only correct if we define density as weight per unit volume. It will work in a system where "pound" is used both as a unit of mass and of force, so it's better suited to jimhebert's needs.
To jimhebert: you have worked out how to calculate the total weight on the floor of the aquarium. Divide this by the area of the floor in feet and you will get the pressure, in pounds per square foot. The pressure at a point on the wall just near the floor will be almost the same as this. The pressure on a point halfway up the wall will be half this pressure and the pressure at the very top will be zero.

No, I haven’t calculated the total weight on the floor. The way to do that would have been 16x4x3.5 = 224 cu.ft.
Then 224x62.22 lb/cu.ft = 13,937 lbs. Total
Therefore, 13937 divided by the area of the floor (64) =218 lb/sq ft.
So based on the last part of your explanation, the lateral force pushing out from a point 2 feet deep 2/3.5 x 218 = 125 pounds/sq ft.
Those are the correct units as far as the math is concerned, but not the correct units for for the answer to the problem. The answer has to be in pounds.