Fibreglass Water Tanks
Fibreglass water tanks (Glass Fibre Reinforced Plastic) can be found on the market for storing rainwater. They are often comprised of a resin, mixed with a hardener with glass fibres that are sprayed into a mould. A food-grade coating exists on their interior surface so that they can safely store drinkable rainwater.
The fibreglass material is very stiff and rigid which allows the tank walls to be created fairly thin. That said, being quite rigid material it can become brittle and crack when exposes to the elements over time resulting in leaks.
Fibreglass tanks are more translucent than other tanks which allow sunlight to enter in. This isn't good since such can cause algae growth, so as a solution many fibreglass tanks are made with sufficient pigment. It is something to be aware to however when purchasing this type of tank.