Essential to any custom water-cooling setup, the reservoir holds the extra liquid in a loop and is designed to allow for air bubbles to slowly be replaced by liquid as it circulates. They also serve as the filling port for when you need to add or replace your liquid coolant. There are two types of reservoir - they can be installed as a standalone unit so you can use a pump of your choice, or as part of a pump/reservoir combo. A standalone unit is usually used in high end builds - if you decide to go down this route you must make sure you plan your loop so that the reservoir is always before the pump.
