Arctic Armor In-Ground Skimmer Guards are designed to plug your skimmer and act as a combination of a plug and a skimmer bottle to prevent damage to skimmers caused by water in the pool skimmer freezing during the winter. The Skimmer Guard is designed to absorb the expansion of the water as it freezes in the skimmer. The freezing water will actually compress the Skimmer Guard instead of expanding outward and cracking your skimmer. The 16" Skimmer Guard is able to fit either 1.5" or 2" internal threads for easy installation and are intended for use in in-ground pools only.