HMS Horizontal Rail (Arm Tank Mounts, and Fixture Brackets Sold Separately) AI's HMS Rail Mounting System Great for mounting multiple lights on one rail across your tank. The Hydra Mounting System (HMS™) is designed to provide an easy to use, adjustable tank mount solution for the Hydra TwentySix, Hydra FiftyTwo and Hydra HD LEDs. Each System Requires: A double arm mounting tank bracket, a horizontal mounting rail to fit the length of your tank, and brackets for each Hydra (sold separately). * for aquariums up to 60". rail measures 61.75"