Automatic irrigation systems, as well as more advanced hydroponics or areoponic setups, are all based on the same principle. The nutrients solved in water are kept in one place and distributed to the plants in regular time intervals and according to the specifics of the particular technique.
In order to protect the nutrient solution and guarantee its highest quality, it is important to store it in a proper manner. Mould and algae are natural enemies of cannabis plants - we don't want them in the growing area. To prevent them from appearing, the reservoir with nutrient solution must be kept isolated from light.
The size has also to be adjusted to the needs of your system. Checking the volume (how many liters it can store) is not enough - you also have to make sure it fits your grow area. Height should also be controlled, as the water table cannot be neither too low (pumps have limited capacity) nor to high (the water might flow gravitationally, overflowing your plants).