These fixtures are constructed to fit glass aquariums and enclosures to position the heat light under the screen tops eliminating the need to remove the lights every time you want to go into the enclosure. They also reduce the wattage of the bulb required to provide an adequate basking temp without heating the entire enclosure, Glass enclosures are frequently poorly ventilated and retain heat. They are adjustable to fit any width of aquarium or enclosure up to 24" wide, and come in two sizes. One is equipped with a 1/2' drop for glass reptile enclosures and reptile "breeder" tanks which are typically around 18" tall, and in a 3" drop for "fish" aquariums which are typically around 21" tall. It also includes an outlet so you can plug in your UVB fixture and run them both off of a single timer.