The most common materials used are Polyethylene (plastics), concrete, fiberglass (FRP/GRP) & steel (stainless steel, carbon, etc).
Polyethylene water tanks (Polytank) are a multipurpose water tank suitable for drinking water & various types of liquids. It is commonly used in water & waste treatment plant as well as oil & gas, agricultural & pharmaceutical companies.
High-density polyethylene (HDPE) as the name implies, has a high strength-to-density ratio compared to normal PE water tank. It is more durable with better heat & corrosion (rust) resistant.
Fibreglass Reinforced Plastics (FRP) or Glass Reinforced Plastics (GRP) is a strong & lightweight which makes it easy to maintain with long lifespan. FRP tanks are cost effective due to its easier installation & maintenance. It reduces the heat transfer to the water as well.
The water tank should be cleaned at least once a year to ensure there is no harmful residual that affects the quality of the water. You can clean it once every 2 year if you have a proper filtration system in place.
Water tank that is not cleaned regularly becomes a breeding ground for bacteria & contaminate the water stored. Clean your water tank regularly to avoid diseases caused by contaminated water.
You can simply patch the crack from the outside. If the crack is more serious, you'll need to drain the water from the tank and patch both inside and outside of the tank. This is a temporary fix and will not last long. If you have too many cracks, it is best to replace the tank.