If you are installing the Shadow® Overflow system on an aquarium with a thickness of less than 1/4″ or has plastic trim, you may find that sometimes the plastic trim may interfere with the rear lid. If this is the case you can use these simple Spacer Rings to offset the rear box from the aquarium. This also allows you to drill your aquarium higher and allows for clearance of the plastic trims. These are designed to fit perfectly with the Shadow Overflow. Sold as a kit with 2 Acrylic spacer rings and 2 extra rubber gaskets.