What type of base do I need for my water tank?

You need a stable base that extends beyond the diameter of the tank and will not be eroded by wind or rain.

As a rule, you require a stable base that extends beyond the diameter of the tank and will not be eroded by wind or rain.

Although there are other types of suitable foundations for poly, Coerco poly water tanks can be secured on top of a sand or gravel foundation between 75 - 100 mm (3 to 4 inches) thick, free of debris, flat, compacted, and approximately 500 - 600 mm ( 20 - 24 inches) bigger than the diameter of the tank. 

See more about water tank installation here:

An In-Depth Guide to Preparing the Ideal Base for Your Rainwater Tank

What Do I Need To Do For Tank Pad Preparation?