This is a special sized expandable plug that is used for the Pentair Aqualuminator Aboveground pool light. Specially made for a perfect fit, this expansion plug fits the Aqualuminator Light. Also fits the Darkbuster and Quasar above ground lights. This Aqualuminator plug will eliminate water loss through the wall light fitting during winter season. Easily remove the bulb from the housing and insert this expansion plug.