The Ultra Zap Bio Filter Lid and O-Ring is a replacement part for a koi fish tank filter. The lid is placed on top of the filter and helps to keep the water inside the filter while also preventing debris from entering. The O-ring is a rubber seal that is placed between the lid and the filter to create a watertight seal and prevent leaks. This replacement part is essential for ensuring the proper functioning of the filter and maintaining clean and healthy water for the fish in the tank.