Water

All life as we know it depends on water, so it is essential for all life support. It is also very commonly used in the production of Food, and even in some Power systems. Unlike e.g. Oxygen, however, there is no "overhead" for the size of the station. Also unlike Oxygen, should Water demand ever exceed supply (e.g. due to an area becoming isolated from Water systems), aliens will slowly dehydrate over time, becoming very unhappy as they do, before either leaving or dying.

The exception to the latter is supporting aquatic aliens: Their zones need to be filled with Water. While the game will abstract away the "filling" process as part of the construction costs, these areas do require a supply of water (e.g. 1 Water per 10 tiles, rounded up). In this context, Water supply represents filtering out waste, managing pH and salinization levels, etc.; as a result, while aquatic aliens will still dehydrate if supply is outstripped by demand, they will do so a lot slower than others.

Additionally, zones designated for aquatic life support need to be reinforced to handle the additional pressure and mass of the contained water.