Marineland/Aquaria Aquarium Light Timer, an easy-to-program, 24-hour programmable device that turns aquarium lights on and off automatically.24 Hour Light Timer is designed for use with all freshwater and marine aquarium lighting, the Aquarium Light Timer features a manual override and is grounded for operational safety. Rated at 15 amps, it has almost twice the current capacity of other light timers.Timer-regulated lighting helps control algae growth, provides fish with a dependable, stress-minimizing waking/sleep cycle, and is ideal for maintaining a plant-healthy environment while the hobbyist is away.Programming is a simple three-step process. Set the current time by rotating the numbered dial until the selection arrow points to the proper number. Set the working time by pressing a grey dial clip for each hour desired for operation -- each clip represents 15 minutes. Then, just set the TIMER ON, plug the light into the timer, and the timer into an electrical outlet.