A common question facing many hobbyists is how long should I run the lights on my saltwater aquarium? The answer is, it depends. For fish only aquariums with live rock (FOWLR) you can oftentimes run the lights for as long as you would like without fear of rapid algae growth. This is because normal output or stock aquarium fluorescent lighting is far less intense than the fixtures used on most reef tanks that have living corals and clams.
Most aquarists keeping corals will run their lights for eight to twelve hours per day. This gives the corals enough time to perform photosynthesis during the day and enough time to feed on live food at night. Some corals simply need more light, so you will either need more intense lights or to lengthen your photoperiod. The problem with running your lights longer is it promotes more algae growth. This can make green hair algae, red slime algae/cyanobacteria, film algae and green bubble algae unmanageable. As a side note, the lower the nutrients in your aquarium, the less algae has a tendency to grow. You can reduce the nutrients by using reverse osmosis water and by owning a good protein skimmer or performing routine water changes.
Another popular trend is to create a dawn/dusk effect with your lights. Most fixtures are equipped with two on/off switches to control the blue actinic and white daylight bulbs. These bulbs can be put on separate timers. For example, you may have the actinic blues turn on at 9am and then the white daylights turn on at 10am. As the day passes and the corals perform photosynthesis, the timer could turn of the daylights at 8pm and then the actinic bulbs at 9pm. Your fixture may also have LED moon lights, which could then be turned on with another timer to allow for nighttime viewing and to create a full, twenty four hour lighting cycle.
