If you remember that a 1000 litres of water weighs 1000 kg then it is easy to see how a larger tank will exert a significant amount of pressure on their bases when full. The following is a rough guide and for more detail an engineer should be consulted.

A concrete slab is a suitable base for your tank, it should be a minimum of 100mm thick and have reinforcing mesh embedded in the concrete. The slab needs to be cast on a level and firm surface. The finished surface of the slab needs to be flat, smooth and level, and should be left to cure for at least five days before filling the tank with water.

The slab should be at least 100mm larger than the tank. This is important. The base cannot be smaller than the tank as problems will occur, resulting in major leaks.