In general, you want to replicate a natural day and night-time cycle. Turn on the light when you get up in the morning and turn off the light in the late evening. Most species of fish will need the light on for 10 – 12 hours. You can simplify this task by using a simple household timer to turn the lights on and off. If you begin to experience a build up of algae, however, one of the first things you can do is cut back on the amount of time the light is on.