This Pack of Four Replacement Screens for the Reservoir Fill Port on the Med4 Elite control unit is a terrific addition to or replacement for the Port Screens that came with the Med4 Control Unit. The screens act as a filter for the distilled water that's added to the Med4 via the Reservoir Fill Port. When needing to replace worn or damaged Reservoir Fill Port Screens, these are the screens to be used. Please note, this is a Pack of Four Replacement Reservoir Fill Port Screens only. Wraps, control unit and all other Med4 and Game Ready accessories are available separately. Med4 recommends refilling the reservoirs after draining every 2 weeks. Before filling the reservoir, users should take the time to clear any debris, if any, from the Port Screen. To fill, carefully pour distilled water into the reservoir port opening until the GUI (Graphical User Interface) on the touchscreen registers the water level at 75%. It is OK to keep the Med4 plugged in while filling and refilling. The GUI provides important information on reservoir status as the water is being added. It is cautioned to not use any screens on the Med4 control unit other than Med4 Port Screens. If damage occurs while using a different screen, the warranty may be voided.