Let a be the size of the square-base
Let h be the height of the tank
The tank being homogeneously made in steel, its weigh W is proportional to the volume of the 5 sheets that are necessary to build it.
All the sheets having the same thickness, the weigh of the tank is proportional to the total surface S of the 5 sheets.
Now you just have to differentiate W with respect to a to find the value of a that minimizes the weight.