When you ask for a specific size tank, there is some suppliers who will promise you more storage volume than what the tank can actualy hold. Check the volume of the tank according to the height and diameter given and remember to subtract the freeboard - this is the top section of the tank up to the overflow, which cannot be filled.
Any tank manufactured for the purpose of storing water should be designed to hold water, but the most concerning factor is when the tank is empty. The tank must be able to with stand wind speeds of up to 100km / hour when empty or else the tank will collapse like a Coke can in high wind speeds.
Steel tanks need anchor brackets that is bolted to the tank wall where the panels join at the base of the tank, to prevent the tank from overturning in high winds. There is some tank manufacturers who does not have these brackets and you risk the tank from collapsing in when empty.
The tank must be secured to a engineer-designed and approved steel-reinforced concrete Class 25 20 MPa ring beam or slab base to stabeslish the tank and prevent rust from direct contact with the soil. Tanks installed without a ring beam will not be able to withstand high wind speeds and will start to rust when in contact with the soil.
Tank design must confirm to SANS10160:1 - 3 (loading code) both in terms of the design of the dome roof as well as that of the tank's cylindrical shell. There is tank suppliers who does not conform to this standard or even use the service of a professional civil/structural engineer risking the tank to collapse.
It is common knowledge that the bolts take up most of the stress due to the water pressure and need to have an ultimate tensile strength of 800 Mpa, the design codes limits the maximum stress to 200 Mpa. Make sure the tank you buy use the correct strength bolt according to the design specification.
The hollow tube section used extensively in the dome roof must be of commercial grade galvanised material and are therefore assumed to have yield strength of just 200 Mpa. Roofs not conforming to these yield strength propose a serious risk to the tank owner when the roof collapse while walking / working on the roof.
The purpose of the plastic internal liner is to prevent the water from touching the tank wall and start corroding. There is low quality liner manufactures who only coat the liner once leaving very small pin holes that will start leaking over time. Water trapped between the tank wall and liner cannot evaporate and will start corroding your tank and reduce the life expectancy of your tank considiberaly.
To gain access to your tank or tank roof you will need a cat ladder. To conform to the heath and safety standards of South Africa any ladder for human use must conform to the SABS standards. Any tank higher than 1,2m must have a ladder with protection rings preventing a person from falling off the ladder or even killing himself. Traffic to the roof must also be restricted.
Most of the tank manufacturers or supplies will guarantee there product for +/- 10 years and have not been in business that long or have a fixed business address. It is not possible to guarantee material used in the tank if the original manufacturer did not guarantee it, therefore you will have to rely on the manufacturers credibility and performance.